Thank you for the advice, Angus. I really appreciate it. I will set up another Discourse instance on an experimental server that won’t affect my other services and try again following your suggestions.
I’m not sure which layer of my infrastructure is where the problem really lies, but it may be the reverse proxy.
I’m using Cosmos Server as a server monitor, docker container management UI, and reverse proxy for my other docker containers like Discourse and other services. I feel like the reverse proxy may need to be similarly configured for adequate rate limiting for incoming ActivityPub connections.
It may have been the incoming ActivityPub sync requests from the external Mastodon server that overwhelmed the reverse proxy and caused the RAM, CPU, and network usage to max out and overheat.
I will update again once I receive my next shipment of Raspberry Pis and have a spare board to use as an experimental server.
Thanks for your suggestions!