I am sharing KL SETV advice, if appropriate..
On application at KL, I had one SETV plus extension in my UK passport.
I took their advice on the website seriously. I filled in the form and the checklist prior to arriving and I got there early, at 08:45, I was already 50 in the queue and they only deal with 130 a day, as I was told.
If you don’t have onward travel booked with proof prior to day 60, proof of your booking\itinerary in Thailand, including copy of housebook and Thai ID if staying with a Thai resident (which I am, but no TM30 asked for to prove my previous 3 months) or if you don’t dress appropriately, you will get turned away.
They don’t want to listen to how you’re going to apply for an extension; that’s a privilege if it’s given, not an absolute. I saw many people get turned away for trying to discuss that as a fact.
That aside, I found it seamless and easy. They didn’t ask for proof of funds or any explanation other than the paperwork and checklist and your POOT.
If this is not appropriate then please delete!