There is virtually no difference between outlook and gmail. Your deciding factor should be the user interface of the web client and the app.
Also, if you are talking spam filters forget about privacy altogether. The whole idea of a spam filter is to have a software that reads your email and decide what belongs to the pile of junk and what is interesting. If it doesn't read your emails it can do that, but if it reads your email you have no privacy.
A blade of grass is quite a stretch. We are talking about emails and it doesn't take much to be reliable. Stuff like Libero and Virgilio (two small italian providers) are much older than gmail and outlook but they are still rocking no problem. Even if their original company doesn't exist anymore users never had to worry about expired domains or anything like that. It can happen, but if you are talking about protonmail I wouldn't worry about it.protonmail is a blade of grass compared to google or outlook.
Forget about it. The email protocol is fundamentally flawed and providers claiming to offer privacy, security and end-to-end encryption are full of shit; protonmail and tutanota included, although they at least try... and try to sell it as such. Thankfully, most of the time privacy and security is not essential so, even if security would be nice, it's not a tragedy if someone tracks you. When you really need privacy and security you'd be better relying on a different service and find the most appropriate way to establish a secure channel that both ends are willing to trust.privacy and always security but on the account so encryption-end-to-end things like that?
Also, if you are talking spam filters forget about privacy altogether. The whole idea of a spam filter is to have a software that reads your email and decide what belongs to the pile of junk and what is interesting. If it doesn't read your emails it can do that, but if it reads your email you have no privacy.