A central relay/proxy is even worse than your current approach. People are obviously free to set up their own Lemmy/Mastodon server if they want a relay.

Lemmy/Mastodon are quite heavy to set up if all you want is to proxy outbound connections. Just using any available proxy you have (which could very well be eg.: a SOCKS proxy set up on FoxyProxxy) is quite nimble and takes up at most a few kbs of RAM.

That said, for anonymizing the IP origin this only mostly works if enough people use the same general relays (basically the same principle as TOR, VPNs), which means this only becomes effective once enough people use this plugin that it becomes worthwhile to position such infrastructure.