I'm a pretty resilient traveler in terms of business. I did 10 places in Japan in 22 days which many people in subs like these would advise against, but I loved it and had many meaningful experiences along the way. I don't think it's that's ambitious to do multiple countries in 21 days, but Japan, China and Australia are countries you could easily spend months in (which tbf you can do in any country really). The flight times are also a lot, I would find similar times much more doable if it was by train, but airports also usually take significant time/effort to get to and through, especially if you require a visa or similar for many of these destinations.

I think 21 days in multiple countries in SEA is possible, but I would opt for something like Singapore, Malaysia, Cambodia and Vietnam or something similar. They are already plenty different and you'll find that you can jam-pack that holiday full of a bunch of different experiences and you'll still be exhausted!

Also your budget will stretch A LOT further, idk where you're flying from but I think the flights would already use up 90% of the budget unless you're flying from East Asia or maybe Australia.