First, Now I've noticed that during the deploy I get:

Caution! After loading partitions, the CRC doesn't check out!
Warning: Invalid CRC on main header data; loaded backup partition table.
Warning! Main and backup partition tables differ! Use the 'c' and 'e' options
on the recovery & transformation menu to examine the two tables.

Warning! One or more CRCs don't match. You should repair the disk!
Main header: ERROR
Backup header: OK
Main partition table: ERROR
Backup partition table: ERROR

Invalid partition data!
Information: Creating fresh partition table; will override earlier problems!

Warning! Mismatched GPT and MBR partition! MBR partition 1, of type 0x82,
has no corresponding GPT partition! You may continue, but this condition
might cause data loss in the future!
The operation has completed successfully.
+ rm -rf /tmp/tmp.Befxfm9cFt
Connection to 192.168.68.103 closed.

First, Now I've noticed that during the deploy I get:

Caution! After loading partitions, the CRC doesn't check out!
Warning: Invalid CRC on main header data; loaded backup partition table.
Warning! Main and backup partition tables differ! Use the 'c' and 'e' options
on the recovery & transformation menu to examine the two tables.

Warning! One or more CRCs don't match. You should repair the disk!
Main header: ERROR
Backup header: OK
Main partition table: ERROR
Backup partition table: ERROR

Invalid partition data!
Information: Creating fresh partition table; will override earlier problems!

Warning! Mismatched GPT and MBR partition! MBR partition 1, of type 0x82,
has no corresponding GPT partition! You may continue, but this condition
might cause data loss in the future!
The operation has completed successfully.
+ rm -rf /tmp/tmp.Befxfm9cFt
Connection to 192.168.68.103 closed.

Back to your answer, It just hangs. No boot screen or anything

It could be the EFI partition since it doesn't show "Linux boot manager" (or whatever was it) in the boot menu.
That option is already set:

  boot.loader = {
    systemd-boot.enable = true;
    efi.canTouchEfiVariables = true;
  };

Can't do virtualisation.vmVariantWithDisko = config.virtualisation.vmVariant; right?
I get:

error:
       … while calling the 'head' builtin
         at /nix/store/ns30sqxb36k8jrds8z18rv96bpnwc60d-source/lib/attrsets.nix:1701:13:
         1700|           if length values == 1 || pred here (elemAt values 1) (head values) then
         1701|             head values
             |             ^
         1702|           else

       … while evaluating the attribute 'optionalValue.value'
         at /nix/store/ns30sqxb36k8jrds8z18rv96bpnwc60d-source/lib/modules.nix:1227:5:
         1226|
         1227|     optionalValue = if isDefined then { value = mergedValue; } else { };
             |     ^
         1228|   };

       … while evaluating the option `virtualisation.vmVariantWithDisko':

       … while evaluating the option `disko.devices._config':

       … while evaluating the option `disko.devices.bcachefs_filesystems.main_bcachefs._config':

       … while evaluating the option `disko.devices.bcachefs_filesystems.main_bcachefs.subvolumes."@backups".mountpoint':

       … while evaluating definitions from `<unknown-file>':

       … while evaluating the option `virtualisation.vmVariantWithDisko.disko.devices.bcachefs_filesystems.main_bcachefs.subvolumes."@backups".mountpoint':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: The option `virtualisation.vmVariantWithDisko.disko.devices.bcachefs_filesystems.main_bcachefs.subvolumes."@backups".mountpoint' is defined multiple times while it's expected to be unique.

       Definition values:
       - In `/nix/store/yq3p8dyba5bkxb7nv2kdf638ak2xnimi-source/hosts/common/global/vm-test.nix': "/var/backups"
       - In `/nix/store/yq3p8dyba5bkxb7nv2kdf638ak2xnimi-source/hosts/fehu/disk-config.nix': "/var/backups"
       Use `lib.mkForce value` or `lib.mkDefault value` to change the priority on any of these definitions.

I suspect it's related to mountPoint

BTW I've fixed the issues with the bcachefs disko config :)

Can't do virtualisation.vmVariantWithDisko = config.virtualisation.vmVariant; right?
I get:

error:
       … while calling the 'head' builtin
         at /nix/store/ns30sqxb36k8jrds8z18rv96bpnwc60d-source/lib/attrsets.nix:1701:13:
         1700|           if length values == 1 || pred here (elemAt values 1) (head values) then
         1701|             head values
             |             ^
         1702|           else

       … while evaluating the attribute 'optionalValue.value'
         at /nix/store/ns30sqxb36k8jrds8z18rv96bpnwc60d-source/lib/modules.nix:1227:5:
         1226|
         1227|     optionalValue = if isDefined then { value = mergedValue; } else { };
             |     ^
         1228|   };

       … while evaluating the option `virtualisation.vmVariantWithDisko':

       … while evaluating the option `disko.devices._config':

       … while evaluating the option `disko.devices.bcachefs_filesystems.main_bcachefs._config':

       … while evaluating the option `disko.devices.bcachefs_filesystems.main_bcachefs.subvolumes."@backups".mountpoint':

       … while evaluating definitions from `<unknown-file>':

       … while evaluating the option `virtualisation.vmVariantWithDisko.disko.devices.bcachefs_filesystems.main_bcachefs.subvolumes."@backups".mountpoint':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: The option `virtualisation.vmVariantWithDisko.disko.devices.bcachefs_filesystems.main_bcachefs.subvolumes."@backups".mountpoint' is defined multiple times while it's expected to be unique.

       Definition values:
       - In `/nix/store/yq3p8dyba5bkxb7nv2kdf638ak2xnimi-source/hosts/common/global/vm-test.nix': "/var/backups"
       - In `/nix/store/yq3p8dyba5bkxb7nv2kdf638ak2xnimi-source/hosts/fehu/disk-config.nix': "/var/backups"
       Use `lib.mkForce value` or `lib.mkDefault value` to change the priority on any of these definitions.

I suspect it's related to mountPoint

BTW I've fixed the issues with the bcachefs disko config :)

Can't do virtualisation.vmVariantWithDisko = config.virtualisation.vmVariant; right?
I get:

error:
       … while calling the 'head' builtin
         at /nix/store/ns30sqxb36k8jrds8z18rv96bpnwc60d-source/lib/attrsets.nix:1701:13:
         1700|           if length values == 1 || pred here (elemAt values 1) (head values) then
         1701|             head values
             |             ^
         1702|           else

       … while evaluating the attribute 'optionalValue.value'
         at /nix/store/ns30sqxb36k8jrds8z18rv96bpnwc60d-source/lib/modules.nix:1227:5:
         1226|
         1227|     optionalValue = if isDefined then { value = mergedValue; } else { };
             |     ^
         1228|   };

       … while evaluating the option `virtualisation.vmVariantWithDisko':

       … while evaluating the option `disko.devices._config':

       … while evaluating the option `disko.devices.bcachefs_filesystems.main_bcachefs._config':

       … while evaluating the option `disko.devices.bcachefs_filesystems.main_bcachefs.subvolumes."@backups".mountpoint':

       … while evaluating definitions from `<unknown-file>':

       … while evaluating the option `virtualisation.vmVariantWithDisko.disko.devices.bcachefs_filesystems.main_bcachefs.subvolumes."@backups".mountpoint':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: The option `virtualisation.vmVariantWithDisko.disko.devices.bcachefs_filesystems.main_bcachefs.subvolumes."@backups".mountpoint' is defined multiple times while it's expected to be unique.

       Definition values:
       - In `/nix/store/yq3p8dyba5bkxb7nv2kdf638ak2xnimi-source/hosts/common/global/vm-test.nix': "/var/backups"
       - In `/nix/store/yq3p8dyba5bkxb7nv2kdf638ak2xnimi-source/hosts/fehu/disk-config.nix': "/var/backups"
       Use `lib.mkForce value` or `lib.mkDefault value` to change the priority on any of these definitions.

I suspect it's related to mountPoint

BTW I've fixed the issues with the bcachefs disko config :)

hm,

    nodev."/.rw-etc" = {
      fsType = "tmpfs";

      mountOptions = [
        "defaults"
        "noexec"
      ];
    };

fails at unmount, because disko calls findmnt tmpfs /.rw-etc when it should be doing findmnt none /.rw-etc