PhD after industry. Is it reasonable?
I did my PhD in my fifties, after a computer science industry career. I am also a permanent resident alien. I can see two problems you are going to have that I avoided:


I didn't start my PhD until after I had my retirement fund saved up, so my standard of living did not change.
I continued in computer science, so my industry experience helped. I could read current computer science papers without too much difficulty.


One approach to the standard of living issue is to rehearse by trying to live for a year at a graduate student spending rate. That will let you find out if you can do it, and build up your investments with the money you are not spending.

The biggest problem I see is the transition to Mathematics. You could perhaps use the Mathematics GRE subject test to evaluate where you are relative to what you will be expected to know. You might do better doing a computer science PhD but specializing in theory. There are some fine open problems, including the great P=NP question.