My Thai wife and I (UK pp holder) have homes in both Bangkok and upcountry. Previously I just extended based on marriage in Bangkok's Cheang Wattana. Would there be any issues to contend with extending at another immigration office close to our other home upcountry where I have never tried before?

My wife cannot come to Bangkok due family commitments, so I want to extend my stay at her home province. I assume like in Bangkok I visit within 30 days of current expiry date, and return for the new yrs extension? Since I work elsewhere in Thailand I need to schedule the visits accordingly.

You have to extend where you live (with your wife). If you have proof of living there you can try, but they migth visit you at the other house and interview the neighbours on your first extension there.

Thanks....