The biggest thing I learned is give postgres a crap ton of ram especially if you use autosubscribe to communities bot. Nginx logs can tell you response times and timeouts, timeouts will be from slow queries and if you get a ton, need to bump up your RAM.
Use https://phiresky.github.io/lemmy-federation-state/site to troubleshoot federation issues.
VPN would still work for iPhone I imagine. Small whitelist of DNS would do 90%+ of the job.