Thai language courses are not that long and yes you would required to be fluent before you can taking courses/going to a university. The prestigious Universities like Julalongkorn , Thummasat offering international courses which would qualified you for the visa.
Do you have Thai birth certificate? If yes, go back to Thailand; to your mother’s area. With two witnesses, your mother and her household registered book , asked to be added to her book and then get your Thai ID. Overseas embassy dose not issue the first time ID.
One consequence you are as male is the soldier duty. Hope you are aware of this . It’s not fully pardoned until the age of 50 , not 30.
The same difference; I did all of my immigration paperwork by myself, my PR and ultimately the USA. citizenship . From my point of view all immigrations process are very transparent and straightforward, no need to pay anyone to do it for you.
Doing all of it on my own I’ve learned and able to did it for my two children. That save me ten thousands dollars.