Question Should I reuse my 7 year old h100i or use the stock Ryzen 3600 cooler?

The main question is how it's going to last. AIO pumps often fail after just a few years, so 7 years is probably already pushing it. You could use it if you don't mind the potential downtime from having to swap it out when it eventually fails, though that could be said for new AIOs as well.

Also, I'm not sure on socket AM4 compatibility. I believe the original H100i might work with AM4, but some of the somewhat newer revisions required a compatibility bracket. This might be the original version, so it may work without additional hardware though.