Fastmail has been about the same for me with speed of response. I don't have to reach out to them often, but when I do it's always quick.

They also don't have a limit on domains. I haven't seen any price gated functionality or any limits. I don't have 12 domains, but I do have a handful and it all works quite seamlessly.

I have been with Fastmail for probably a decade or more now. Worth it.