World simulation (ticks) is single-threaded, but things like world generation are multithreaded. I’d recommend Paper as server software as it’s more performant out of the box (vs. vanilla) and configurable (ex. how many threads world generation is allowed to use).

If you host multiple worlds I recommend spinning up a Paper instance for each world separately and connect them with Velocity.

Ryzen 7000 should have better single-threaded performance than your i5-9500 but as it’s a VM ymmv depending on whether Sparked Host overprovisions their machines.

Couple of years, yeah.

They run their own registry at lscr.io. You can essentially prefix all your existing linuxserver image names with lscr.io/ to pull them from there instead.