I’ve been using OVPN with port forwarding for a couple of years, it works great for my use cases.

I haven’t used other providers, so I can’t really compare to others though.

That’s not really a Proton Mail limit - if the mail is too big, it’ll just be rejected by some other mailserver on its way, even if Proton allowed it through.

And Proton would get a lot of support cases from people asking why the mail they sent to a Gmail account was returned…