I doubt you would have to accept their anti-virus even if they did launch one, there are many ISP’s that offer an AV, simply because most spam email is generated by systems that have been infected and taken over to act as spambots. They might well scan your email on the server, but they can’t scan your system without your installing the software.

I certainly wouldn’t jump ship because my ISP decided to use secure email at least they are using the correct secure ports (465 and 995), some just use the regular ports and that causes more of an issue. If you had other issues with your ISP then that would be cause to jump ship but not this in isolation. Once set-up STunnel works well with avast well with avast.