OP, being UK you could do as @Mutt Daeng suggested and obtain a non O eVisa.
Based on marriage or retirement (if 50+)
That option would avoid and dealings with immigration.
Yes visa exempt entry is currently 60 days and can be extended by 30 days at immigration.
That would require a TM30 and 1900b cost of extension.
Airline at departure may require an onward flight within 60 days.
You have a return ticket (same airline?) should not be an issue with explanation that you are obtaining 30 day extension.