CloverBootloader VS BootDuet

Definitely look into clover

You need to try and learn it ....clover is a bit eazy then OC https://github.com/CloverHackyColor/CloverBootloader/wiki/Configuration

I did manage to get Clover to boot, turns out it was a graphics card issue (Clover does not boot on ProLiant DL 560 Gen 8) so I threw an old HD 3470 I had laying around in it and I can now access Clover. I gonna play with Clover a little and see if I can get it to do what I want and if not, I grab the Slim ODD to SATA adapter you mentioned. Just need to find the cables to connect it.

In your case you may wish to research using a tool such as Clover Bootloader, which would allow you to install Proxmox to NVMe drives and then put Clover on an internal or external USB thumb drive. The boot process would start from Clover on USB before linking your NVMe installation and continuing from there.

Clover bootloader https://github.com/CloverHackyColor/CloverBootloader

You can use Clover to emulate UEFI on legacy BIOS computers.

Are there any other bootloaders I should be considering? My understanding was that LILO and BURG have essentially been abandoned so even if I were willing to rely on an abandoned project for something as important as the bootloader (I'm not btw) then it probably would not work with LUKS/Btrfs. While doing my initial research, I did come across Clover but I know even less about that than I do about Systemdboot and Refined. My initial impressions is that it seems very customizable theming-wise but it feels like maybe not too many people use it / I'd have a hard time finding help and that's IF it even supports LUKS/Btrfs.

You Clover outdated af (2017). Update to 5123.1, which is the last one with the old memory fixes: https://github.com/CloverHackyColor/CloverBootloader/releases/tag/5123.1

UEFI emulation is a thing.

For people with legacy-only firmware systems, you too can run UEFI on lazy cloud providers and legacy hardware. I do this on my 14 year old dell laptop just so that all my x86_64 systems have the same boot efistub linux kernel images. All you need is to use one of the EDK DUET bootloader builds such as BootDuet¹, or for an easier user experience CloverBootloader². Only complaint is secureboot emulation can be a pain. But I imagine with Windows 11 around the corner requiring TPMs, emulators with this feature will probably get more popular.

¹ https://github.com/migle/BootDuet