Tutanota has been pretty decent to me, though their android client is sometimes a little rough around the edges. Though never in a way that's caused problems for me or impaired usage, and it has a pretty enough interface.
They have a free plan if you're broke as shit like I am and are looking to switch from a free proton plan
Grayjay can work that way (optionally) too. Its pretty cool there are now a number of great ways to interact with YouTube that give users more agency! 😊
Invidious is also a thing, though I haven't used it much
My experiences with pop haven't been amazing (just always ran into small quirks and issues that shouldnt be a thing, but I haven't messed with it all that much), but I'm pretty happy for there to be another big polished desktop environment with resources behind it, I think it makes the linux ecosystem better :)
I've done this with my pixelfed/Mastodon to post on lemmy for easier image hosting. I still can't get over how cool it is that you can essentially "post to 'reddit' from your 'twitter' or 'instagram'"
There's a Foss dating app called alovoa, I think I saw something on the subreddit when I was taking a look at it again a while back about potentially implementing activitypub
Happy lemmy anniversary, we're glad you're here! 😊❤️