I use Protonmail as they offer quite a bit of privacy, offer lots of features, and are free. I particularly like that they protect from tracking pixels notifying the sender you read their email.
I don't know if it'll ever "get there" because they've not demonstrated that they're putting in any effort at all. It's currently on version 3.0.16. According to ApkMirror, they released 3.0.1 in June of 2022. So that's over a year of "stability improvements and bug fixes". Meanwhile they launched Proton Pass, Proton captcha, desktop clients for Drive, introducing new plans, etc.
Honestly, I just want threaded conversations on mobile.
Indeed, same here but while using my own domain name, this way if some day, for whatever reason, either they go "bad" or I find "better" I can switch without much worry.
The service is absolutely free. That feature is not.
Local and offline desktop mail clients in 2023 are now niche features, especially when the service we are discussing gives you a super solid webUI and mobile apps (which also work offline). The rain why Protonmail has this as a paid feature is because only a small subset of it's users even need this as a feature, and it requires a special network bridge to make possible.