I'm also a premium subscriber, and have struggled with the same issues on the iPad app. I try to keep some shows downloaded for my toddler, and the download feature never seems to work on the first try.

I finally got so fed up, I bought a Samsung Galaxy Tab A7 off ebay for $50 and flashed it with LineageOS. I can now load whatever media I want onto the 1 TB sdcard I've installed in it. The 5 year old hardware plays videos just fine with the VLC app. And, as a bonus, I discovered that NewPipe, an alternative YouTube client I installed through the F-Droid store, is actually much more reliable at downloading videos than the official client. I was planning on using yt-dlp to load up the sdcard, but now I don't even need to do that.