https://github.com/mreid/feed-bag

Not sure if it does all you want, but the basics are there, and it wouldn't be beyond the pale to make something like this do what you want. The code is pretty clean

No but I have an indirect answer (a method?) for you. There are many open source projects that do this type of work. For example, newsblur. Maybe you can find a few of these projects in the language you want to use and see how they're handling it. I would expect the to be done common libraries used between them.

I don't know of a project that does this, but if I was to tackle it I would convert the RSS to the Activity Streams standard - https://www.w3.org/TR/activitystreams-core/.

Activity Streams are basically the new RSS and it's a lot better than RSS.

Mastodon is built on Activity Pub, which is built on Activity Streams - so you shouldn't even need to touch RSS at all. The AS already exists. You can access it via the API.

Under European laws all services are required to give you a copy of all data associated with your account if you ask for it. And Mastodon being a European product is of course fully compliant. Just go to your profile and hit the "Request your Archive" button. You could do that once a month or something.

I use miniflux, and you can configure it to modify feed items. As far as I know it does not purge anything by default.

Really, pulling an RSS feed and parsing it, storing stuff is probably 50 lines of bash, and less in a general purpose scripting language.