Sharing my experience while extending my visa today at Muang Thong Tani.
Arrived around 1PM had to fill out the forms and queue. If you have the forms ready, it will save you time. Everything including waiting time was probably around 2++ hours but there were a lot of people.
Stamp was given on top of my current stamp and not from today. 1st covid extension and got quite lucky on this one.
Documents shown:
embassy letter
tm30
all the copies needed for extension
all the forms needed for extension