If you create a app, you can choose if it will be free or not, but the one that i use is foss, i use feeder on my phone (is a android phone and i downloaded the app from f-droid, i don't know if you will find it on app store or if they even have a ios version), and on my desktop i was using thunderbird, but i haven't found a desktop rss reader that i really liked

I use flym decsync.

It can synchronise with other android devices you own using syncthing.

So far there's no windows application RSS that supports this though.