I made a thing for generating full-history RSS feeds for sites that have consistent weekly update schedules and predictable URLs:
https://fh.minilop.net
For example, here's a feed for Doonesbury: https://fh.minilop.net/f/America+Chicago/c3107d1e/https://www.gocomics.com/doonesbury/%25Y/%25m/%25d
And one for Schlock Mercenary: https://fh.minilop.net/f/America+Denver/1c69a1229e/http://www.schlockmercenary.com/%25Y-%25m-%25d
If your feed reader supports RFC5005, these feeds will show every post ever made, not just the most recent ones. But your feed reader probably doesn't support RFC5005... yet?