Just to clarify, the only time you really need to buy an email account under your custom domain is when you’re receiving mail, and I personally prefer Google Workspace.
Sendgrid, once you connect your domain, I believe can send from any email address under your domain. I do want to give you a fare warning about Sendgrid and their poor deliverability. With that being said, I recommend Postmark.
For #2 and #3 do you have some type of help desk like Help Scout? If so, you can just have your emails forwarded from your registar over to the email your help desk provides you. Unless you’re talking about building a help desk type system within your app. That’ll be a little more complicated… Let me know.
The reason I’m using Sendgrid to start is they offer free emails, which is important for me as I don’t really have startup budget.
Regarding #2 and #3, I’ve been thinking about creating my own help desk system within my app and having emails routed to Bubble via some kind of API. Not sure how ambitious that is, but I figured that would give me full control over user data along with ticket system behavior. Is that a bad idea?