I’ve thinking about ditching my Protonmail email address with Posteo since I need IMAP support (because I hate using the Protonmail app) and honestly paying for Proton Mail Plus didn’t worth the money because they are pushing me to buy the Protonmail Unlimited but I don’t need all those capabilities nor would I use them and the best alternative I’ve found is Posteo and honestly I like it, I even got CalDAV support which I didn’t have in Protonmail, but personally I don’t know anybody who use Posteo, so I post this here.
Posteo is great, it’s simple yet has all the features one needs. It doesn’t overwhelm users either with what you mention, bigger better tiers… Other honerable mentions if you’re keen on checking out other mail services are: Runbox, Mailbox and Fastmail.