I haven't tried to leave Gmail, but I've run email servers for 15 years and have never had trouble sending email to Gmail (get plenty of spam from them though). I often read people saying Gmail rejects mail, but I suspect such people are running mail servers on cheap VPS providers, who have bad IP reputation.
IP reputation lists predate Gmail by many years (eg spamhaus). I know Gmail used to use them, and may still. It would obviously make sense to keep their own stats as well.
I've successfully exited Google Workspace for email, calendar and contacts about two years ago. Went with Fastmail. It's been rock solid.
Unfortunately, Drive and Voice are keeping me subscribed to their service despite me wanting VERY BADLY to exit it. Neither have good alternatives that check all the boxes.
