Switching isn’t a problem, you’re protected by the Current Account Switch Guarantee, so even in the unlikely event you have any problems with the switch, you shouldn’t be left out of pocket.
Similarly, you shouldn’t worry about accounts being frozen, for all the reasons others have said.
Can I just ask, why are you wanting to use the switch service, given you’ve been moving payments over manually? Is it because you want to speed up the switching of payments instead of doing them manually? Or is it because everything has been moved over and you want to close the account?
If the latter, I’d suggest there’s no harm in keeping it open, just in case. I’ve kept my legacy account open to make it easier for me to pay in cheques and cash (which I then transfer to Monzo). I don’t much use it otherwise. I’m living proof it’s possible to be Full Monzo while still keeping an old account open.
ETA: forgot to say that one of the biggest reasons I’ve kept my old account is so that I’ve still got at least one VISA card to go alongside all my Mastercards, in case there are any problems with the Mastercard network.