The dark web and security questions

I consider myself fairly knowledgeable about computer matters, but I'm unsure if I could potentially be "at risk" with a gmail account.

Long story short, at some point my favorite gmail was hacked. I found out two ways. First, when trying to sign up for twitter for the first time (I was late getting in on that platform), it said my email had been banned or suspended for suspect activity. I had never signed up before, so I sent a few messages to inquire. Anyhoo, apparently someone outside of the country had used my email address to spam folks or something. I changed my gmail password and lived my best life. After seeing a few people I know get their identities stolen, I decided to purchase "ID theft" insurance through Zander. It was here, during my initial setup that their software flagged my gmail as suspected to be compromised. The details read something about the dark web and such. So, I changed my password again and now I used two factor authentication to log in.

My question is since someone has gained access to my email at some point, can they get back in through a "back door" bypassing the regular login credentials? Is this particular account compromised for good? This is not my primary email address, but it probably is my #2 account. I don't have a lot of sensitive stuff on this address, but I do purchase things using that address. I'm worried that I could be leaving the door open for a full ID theft if someone can access it without me knowing.

I'm also clueless about the dark web. If anyone has insight, please share. Move to tech board if warranted.

Thanks in advance.