

Mailbox.org is a good pick to consider IMO. You can read some comparisons on PrivacyGuides, which I also recommend as a starting point for these sorts of topics. The mailbox.org web UI is not great, but it allows IMAP/SMTP access, so I use Thunderbird on both desktop and Android in order to interact with my inbox. My inbox is auto-encrypted with PGP using their Mailbox Guard thing, so my emails are all encrypted garbage on the web UI anyway. Mailbox.org only allows paid-for accounts, but considering the annoying stuff that Proton and Tuta do to their free accounts I’d rather just be honest about the service I’m getting. It allows auto-forwarding directly in the web UI, but given that you can hook up to it with IMAP anyway, it’s not like you couldn’t just do it yourself.
(Also, as another comment said I also recommend DuckDuckGo’s Email Protection for email aliasing if you need it.)
I’m used to LanguageTool, and at a glance it seems like Harper covers way fewer rules than LanguageTool does. Not sure if this is actually noticeable in practice, but I run my own LanguageTool server and am not too picky about the performance, so I’m not in a rush to move until someone figures out a good way to compare them. LanguageTool’s rules are all open source at least, so it’s only a matter of time before Harper gets anything it might be missing.