I have a true sim in Thailand on rolling PAYG and reduce that to 100 baht a month when I leave. In uk I have an e-sim and on a plan with my son who gets roll over data that he uses and cost me £8 a month and is the easiest way for me. I used to use two phones. That was a pain
I have done exactly what your talking about and when you get back go to the immigration and get the visa transferred and my visa ran out a couple of weeks after entry
well if you get stopped always best to have proof of where it has come from. Border patrol stopped a guy at Heathrow and he had 8k and had to prove where it came from