you can check out sasin business school, which is the best but also the most expensive. chula uni (other departments separate from sasin), mahidol, nida, bangkok uni, thamassat, and a few others, also have masters open to international students, which would be a lot cheaper.
not sure what stage of your career you are at but an MBA, part-time MBA, or EMBA, at one of the thai business schools, might make sense, especially sasin school in chula uni. that would get you a visa for a couple of years, a local network for your business, and i'm sure they could sort you out with some top class thai language lessons.
i think you will have to study thai at a university or do a masters in something else if you want to stay on studying here. thailand is getting tough on language centers helping westerners get visas (ironically over the last few years, many western countries have been cracking down on thais/others doing the same). are you really so rich that you wouldn't even consider getting a job? if no, that's another option. if yes, have you considered elite visa?
it's on the embassy website so you can apply for a certificate of entry, but no doubt more people will apply than they have spaces for. and there is no guarantee things won't change between now and when your flight takes off.
If you want to stay here longer and continue studying, it would probably be better to enroll in a masters degree at a university. Volunteer visas are for volunteers, which is something completely different.
only to the extent that arrivals from countries like the UK are more likely to infect staff or others they interact with on the flight, in the airport, or during quarantine, and/or to pass through quarantine infected but undetected with an extra-long incubation period (there have been cases of over 1 month of false negatives).