It's the same with OsmAnd
I actually wanted to support osmand financially, only at the time the only way I could find to do so was through the play store, which I didn't want to use.
I've tried the simple apps and they are ok, but there are so many good alternatives on fdroid they don't stand out in any way.
If you get Amazon digital credit, e.g. from a "prime day delivery", you can use the fire store to donate that money to Osmand or whatever other app of preference.
Usually the default GrapheneOS/AOSP apps are good enough, but not the calendar. Also I never found a calendar I like better on F-Droid.
it literally is (see `.plus`); in fact, if you install from F-droid, and then open Google Play to look at OsmAnd+, `Install` button is gone, just `Open`