As a Pokemon fan I understand your pain. It's not like it's an obscure series, or from a small company. Why is it so hard to stream such a popular anime? I'm surprised The Pokemon Company hasn't rolled out their own streaming platform yet.
Before diving in to Plex I would highly recommend looking at Jellyfin first also. It's offers much the same features as Plex but is fully free and open source.
For my own media server I use an old HP Microserer G8 purchased second hand, and upgraded with a Xeon e3-1260L, also sourced cheaply used. It's small, easy to service and happily runs my Linux disro of choice. I know other people using various SFF PCs, or even repurposed old desktops. For best performance look for a CPU (or GPU) with hardware video encoding support. Otherwise, the rule of thumb for Plex used to be a CPU with at least 2000 Passmark score on cpubenchmark.net per concurrent 1080p stream.
I've been a longtime fan of CheapShow, a comedy podcast loosely based around unusual items found in cheap shops and charity shops (thrift stores). Episodes include deep dives in vintage/retro media, taste tests of weird foodstuffs, various games and challenges, plus a lot of complete chaos and toilet humour.
Maybe not for everyone, but if any of the above catches your interest it's worth a try.