I actually built a little toy service (that I will also eventually open source) which converts any RSS feed to an ActivityPub actor that you can subscribe to in Mastodon (or any other AP-compatible client).
Play with it if you like! It is SUPER rough and most feeds end up horribly rendered in Mastodon but it's still kinda cool to see it work: https://bots.tinysubversions.com/convert/
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