By the way, in case people are interested in hardware specs for Retrospring:
Even though it is a Rails app like Mastodon, it barely is dependent on background job stuff, so with regular site usage it'll manage upwards of 80k registered users on a 4GB RAM VPS.
https://retrospring.net was running on a tiny AWS Lightsail VPS, with Postgres, Redis, Sidekiq and RS itself on one host up until 40k users. We only switched to our beefed-up cloudscale setup because of the CPU throttling from Lightsail.