Is Ho Chi Minh City a good place to apply for an ED visa for studying Thai and what are the requirements for proof of onward travel?

Hi - hope someone can help me with a bit of insight. I have some tourist visa and one entry on visa exemption (in 2019) and got the red stamp in Vientiane in June. I want to stay in Thailand for another 6 months and actually learn Thai - so going to a language school in CM - now the big question is is Ho Chi Minh City a good embassy to get the ED visa?

I plan to fly in and out of CM to avoid other airports. I know that I need to have proof of cash - and onward travel but here is my second questions - does the proof of onward travel (also for the embassy) need to be within the first 90 days or can it be with 6 months (as I understand the visa grants you 90 days and you can extend in the country by another 90 days).

so 1) Is Ho Chi Minh City feasible for me - and if not where else should I go

2) Is proof of onward travel needed within the first 90 days?

Thanks in advance for all your help

forgot to mention - nationality is Austrian and it would be the first ed visa ever