it takes almost no effort to file an updated TM30, just do it and you can relax. Returning from abroad is almost certainly a basis to refile, tho as mentioned some offices don't care.
Looking through your previous questions, your biggest impediment is that you aren't researched. So spend a few days reading through the posts and replies in this group and take notes.
Most important is that the soft power course has all the government certification and business registration for DTV application approval. The business should be able to prove to you with previous successful candidates and DTV applications. Apparently Muay Thai is cheaper than cooking. Be and choose wisely.
No surprise as every and any extension costs 1900 baht. The unknown factor is whether they will require updated/current paperwork similar to the application via a consulate to process the extension.
Read through the posts and replies to get an idea of which consulate would work best for you. Not being a national of the country you apply from is not an issue whatsoever for 99% of applicants.
How many times a week are we gonna see this exact same post with only the names and locations differing? Buck up and pay an agent if it's too stressful for you. While you're in Pattaya have a beer 🍺 and cry the blues over a ladies drink with your companion.
watch the terminology you're replying to. The OP can't switch anything, there is no visa switching, you cancel one and apply for another. Regardless, the DTV is unavailable in Thailand.