Does my wife need to complete a TM30 upon returning home to Bangkok after staying in Phetchabun?

As mentioned, it depends on the immigration office. I use Nonthaburi immigration which basically follows Bangkok immigration rules and they specifically told me no need to file a new TM 30 unless you leave and re-enter the country.

Get in the habit of doing it online. The first time it's a 3 step process to register an account, add an address then add the foreigner details. After it's set up the third step, which you do each new arrival, takes a couple of minutes.

We have stayed in hotels all over Thailand got TM30’s when needed, never put another one in when we moved

So easy to do if she's done one already. If you use your passport to checkin to the hotel, I'd suggest doing a new one when you return.

Simple answer: yes...

Think it depends entirely on the immigration office you use. I got told of when I went to do a 90 day report because I had been elsewhere in Thailand and hadn’t reported to immigration when I returned home.

I would say, YES, if where you're going inside thailand is gonna file a TM30 on you then when you come back to your primary address file a new TM30

The reason I say that is now that the online 90 day reporting system cross checks with the TM30 system if you're registered somewhere else (even if that place puts a check out date) when you go to file a 90 day report it will mismatch address wise and you'll get rejected. 🙁

If you have a log-in/profile on the online TM30 just file a new one when you get back, export that file and print it so you have a TM30 receipt like this