Yeah they don’t like too many land crossings. The STV extensions are ending soon anyway so you won’t be able to stay longer than your original permitted stay.
The STV extensions are supposed to be ending on 26th November. Alternative option would be to get a 60 day tourist visa and extend 30 days so getting 90 days. Then you could pop into Cambodia and do that process again. Get to see another beautiful country and save yourself the restrictiveness of the STV.
Apply for an extension at immigration office when there. Easy to do and it’s not like it’s really busy at the moment so should be in and out fairly quickly.