Which location is the easiest for applying for a DTV visa outside Thailand, and when will Cambodia require prior appointments?

Go to the website of the wanted embassy, and read the visa section.

It would be good if people could read the forum as these sorts of questions have been answered tons of times already. Cambodia is e visa only beginning next week. You won't be applying there in person anymore. Same thing goes for Ho Chi Minh and Hanoi will also move to e-visas shortly thereafter. Jakarta is having technical problems so will probably not move to the evisa system until November or later (but being Thailand they might make a sudden announcement and move to e-visas without much prior notice). Still, anytime in October seems safe if wishing to apply in person in Jakarta.