CloverBootloader VS MacForgeFramework

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You Clover outdated af (2017).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

SIMBL lives on mostly thanks to w0lfschild, see: https://github.com/w0lfschild/MacForgeFramework. This repo hasn't been updated since 2018; I'm not entirely clear whether the most recent versions of MacForge is still based on SIMBL.

The SIMBL on my website uses an unmodified SIMBLAgent binary taken from an older version of w0lfchild's mySIMBL (https://github.com/w0lfschild/mySIMBL). However, I recompiled his SIMBL.osax with 32-bit support added back in, since I use a lot of 32-bit apps: https://github.com/Wowfunhappy/MacForgeFramework

Quick note that I don't really understand this code, I can just tell you that it works, at least on Mavericks.