If you'd like to turn partial feeds into full-text versions, the project I work on might help. Doesn't work on all feeds, but will work on many: http://ftr.fivefilters.org/
That said, I'm finding that for Substack, at least, RSS works just fine. You get the whole article, no fuss, and it works the way it should, it's just https://example.substack.com/feed.
I'm pleased to see Substack do most things right. I almost wonder if their pitch deck explicitly said "we will zig every time Medium zagged".
There are some non-commercial blogs with no ads, nothing but text on them, but still giving you only the title in the RSS feeds. I suspect it's the default configuration of whatever software they're using for the blogs, and they may not even be aware that RSS is a thing. Maybe I should start writing mails to them to inquire why that is and maybe change their minds on that, since I really can't see a reason for it.
FWIW Most of the feeds I subscribe to are full-text.
A huge annoyance for me too. It's infuriating to me that marketing people are continually breaking things like RSS. It's unfortunate that there's no way for users to give negative feedback to people who abuse a protocol and render it less useful to everyone, which is worse than not adopting it at all.
It's far less convenient, but just running the articles through Pocket makes for a superior reading experience (particularly on the Kobo), since Pocket will extract and display only the readable text, and avoid the nightmare that is trying to display your average webpage on an e-Ink reader browser.