Thank you for mentioning FairEmail, and thank you @wreckedcarzz@lemmy.world, for elaborating on what makes it great.
Thanks to your recommendations I installed it last night and paid the $6 one-time license fee to unlock the advanced features. Being able to set custom notification sounds per sender is a feature I’ve been wanting on my phone for years. I finally have it now and it’s already changing my life for the better.
Not quite as simple as checkboxes, but the ability is there to some degree!