And since you won't be able to modify web pages, it will also mean the end of customization, either for looks (ie. DarkReader, Stylus), conveniance (ie. Tampermonkey) or accessibility.
Buy a domain sure, but just use paid protonmail and use it with the domain you bought. It takes a few minutes to set up, and if protonmail ever goes to shit, you still own the email address. Maintaining your own email server just isn't worth it imo.