Hmmm, interesting "dilemma" ...

Bangkok - stay in a few different areas, see how it grabs you (or not). Silom, MBK area, Sukhumvit. I've never been to Khao San Rd, and don't intend to. But maybe it's your thing. I am sure someone there knows about yoga.

Take the overnight train to Chiang Mai (ideally first class - it's cheap, and not really "first class"). Otherwise fly. It's too far for bus.

Maybe work your way down south. Stop before the provinces adjacent to Malaysia - read up on why you probably shouldn't go there.

eg take the train or bus to Hua Hin. Hang out there for a while. Then maybe one or more of Samui, Phuket (avoid Patong), Krabi.

Or since you need to get to Cambodia, maybe head east instead. Koh Chang ?

Have booking.com on your phone. It will figure out your location and tell you what hotels are around. Book as you go. Use Tripit to remind you where you're going next.