Thanks for your detailed answer !
Nice news for jim ma's loader, i'll follow that (but with baremetal install kinda hard to test if it's the production server xD).
And that was indeed what I though for the cpu and it doesnt change anything by adding a dedicated GPU, right ? (that's what you are trying to by telling me I can't compile additional drivers ?)
And about switching to OMV that could be a great idea, but I'm on SHR partitions and I though it wouldn't work out of synology (maybe I would be able to switch SHR to a raid 5 ? But having same size disk ?)
Thanks for your help !