If you're ok self-hosting, I use www.freshrss.org

Why are you moving on from Vivaldi?

I self host FreshRSS, I can recommend this one easily

FreshRSS with Capy Reader on mobile .

I like FreshRSS.

I've been on FreshRSS for over a year. Works well even on shared hosting. https://www.freshrss.org/

Please add cross-references when you post the same post to two communities at the same time. That way, people can find the other comments and discussion.

I ahve used capy reader on android, available on Fdroid. Currently, I am using Nextcloud news as my RSS, works everywhere, web clients, android clients, all FOSS.

I've been using Feeder https://f-droid.org/packages/com.nononsenseapps.feeder/ Works for my needs, but I haven't explored much.

https://www.commafeed.com/

https://www.reederapp.com/