I’ve been on basically that exact same journey. I made a little iOS app that uses rss feeds and the iframe player to play videos: Unwatched for YouTube (just released it two weeks ago). It’s also open source, if you have any specific questions let me know.
Thanks :) pe1uca summed it up pretty nicely. I‘d add that you can get all your channelIds (which you need for the rss feeds) by exporting your YouTube subscriptions via google takeaway. You can take a look at what the rss feed gives you, it’s as pe1uca said the 15 most recent videos and sadly not all the info: duration e.g. is missing.