> 2. You can use a dedicated SMTP relay service, but these are usually quite expensive ...

I don't typically give endorsements, but I've been using DuoCircle.com since back when they were part of "dyn.com" and I've been very pleased. Apparently they have a free tier, but their current pricing is very reasonable to my eye. (I'm on an old annual plan that doesn't appear to be offered anymore...)

https://www.duocircle.com/email/outbound-smtp

Deliverability through them has been very good over the years.