By any chance, when you did set up the new tablet have you used Google’s method to restore all your settings from your phone, like saved messages, apps, etc.?
You could transfer a malware to the tablet that way. Do you have children who likes to play games on your phone? A dodgy game can be the source of the issue.
Or someone has hacked your wifi/network or you have some malware installed on one of your network connected devices.
Or, you have a Man-In-The-Middle attack. Are you using any (free) VPN services? Then you might want to contact your ISP.
I had before messages from Google telling that someone tried to log in, but not to do recovery. And it was due to some Google applications which were running in the background and were accessing the Google services when I changed networks. But your situation seems to be more serious.
I would change all my passwords (starting with the Wifi one) in your place, but before I would run some Antivirus and Malware search on all my devices. And only secure password, and not reusing them anywhere.
Have I Been Pwned - Check your email address in this database. If it shows up, then definitely change any password which is older than 6 months.