Just passing on a tip from FediTips I didn’t know about. Using the format
(server)/tags/(hashtag without the #).rss
Your RSS Reader can follow hashtags e.g.
https://mstdn.social/tags/dogs.rss
You can see everything with the hashtag ‘dogs’ on mstd.social and every instance mstdn.social can see.
Yes I am in the same boat, but recently resurrected my RSS collection with FreshRSS, the self-hosted application for RSS aggregation. Has many mobile applications, browser-addons etc. Only thing missing is my vast organized collection of websites.