I’m not sure what you mean by “what’s the line” but if it means “what’s the alternative”:

First start using personal domain so that when I switch away I can do it without having to update all contact details. I’ve already been doing this.

Then after some research, switch to a paid email provider. I’m thinking of ProtonMail or Zoho at the moment but I need to look into it more.

"What's the line" as in if you were to draw a line in the sand - what would be "enough" to put you over said hypothetical line of leaving GMail? I think OP may be implying that this should be "enough", but I don't want to make any assumptions.

One thing to note about Proton is if you want to do any shared addresses, say for family use cases, they're missing common features like aliases because of the implied encryption. I tried to switch to it a few years ago and it didn't work for all of the use cases I needed. I do pay for an account with them for other use cases and they're great otherwise. I also tried Zoho a long time ago (>8 years now) and it wasn't good at the time. I've been using FastMail for more than 7 years now and moved all my Gmail domains over about 2 years after that. I have nothing but great things to say about them and aren't affiliated in any way other than being a customer.

Ah thanks for that. That makes sense! I'll also check out FastMail. Thanks for the recommendation :)

Seconding Fastmail. Their great advantage is that they make money by providing email service and their focus is correspondingly different from the Borg.