I also had multiple pis doing nothing. I finally got pihole going on the 3b with DHCP server, hosted my blog on the 4, and have a 5 hooked up to a keyboard, mouse, monitor, and speaker, and when set to 1080p, it's actually a responsive, usable personal computer. I'm developing a three.js game on it that uses a per-frame-updated 512x512 soft shadow map and runs on it at about 50fps at 1080p. I'm planning on running a node.js server on the pi 4 (or maybe 5) and make it multiplayer, though maybe there's a better way to implement the server backend. Of course, if the pi 5 can run it at 1080p50, then so can any random junk laptop from the last 10 years, which is a good feeling.