This guide is probably going to work for you. I wrote this a couple years back and have tweaked a few things last year that needed to be updated. In fact, you’re using a nearly identical system configuration to me;
When I built my system, I was running a 3900x, rx480, 6900xt, with NVMe passthrough for storage.
You can use systemd boot, You just have to change where you edit your config. (it’s in /boot/loader/)
Give the above link a read before we get into a back-and-forth about possible solutions to see if a solution jumps out there first though.