If you extend at immigration you need to show ongoing activity. Currently no requirement to show on arrival at airports but some SPers say they have been asked to show current enrolment
If your docs are fine then no request for more docs made go straight to approval. If they find something wrong you can be rejected or given a shorter term visa.
Why not apply nearby In Vietnam, Indonesia etc since you are in Thailand? Someone recently had their DTV revoked and banned from Thailand for a year for not being in the country where he submitted his application.