for low power you want to go for an intel system or some select amd apu systems.
It depends on power usage though, maybe the best thing is to put a 5700g in your old computer if you don’t have that high power cost (this is a comparison between power cost and cost of hardware). the x570 is still a power hungry system though.
For example, my server can use about 5W where a x570 with 5600X is probably closer to 30-50W. Where i live 50W continious costs 200 Euro per year.
Looking at your post you want to have a relatively powerful system to run proxmox with multiple VM’s and hard drives. I would just build a new system with an intel cpu with multiple hard drives and 64gb of ram. the harddrives are the most expensive part if you want a lot of storage.
In comparison my nas is a intel i5 12400 with 2 ssd’s and 3 16tb hard drives. it uses about 30W idle and this is without parking the hard drives. (the limitation is somewhat in my motherboard). My small virtualisation server uses 20W but i think i can get it down to 5-7W if i stop using the nvme drive.
Transcoding is going to be tricky in a vm! I just caved and use my nas for plex which is just bare metal debian.
( you can check my forum thread here Nutral's HomeLab blog )