Looking for advice... First SE Asia trip, where to start???

[Luko]

If fall is your trip period then you should definitely rule out Thailand and the Phils. on the other hand it's possibly the best season for Indonesia, which I consider as the best diving in SEA too.
Spring gives you two options : Philippines and Indonesia, Philippines sea conditions may be marginally better than Indo .

6kUSD is a wealthy budget even for 4 weeks/20+ days diving and domestic transport.

I would do in Indo :
- at fall season a combination of Bali (10 days), Komodo 6 days (small Liveaboard, check out Dragon Dive Komodo) and Alor 9 days (Alor Eco Divers has single-friendly rates)
- at spring time, Bali 10 days, Raja Ampat in a homestay 7 days , N/Sulawesi (Lembeh, Bangka) 8 days.

 

[Bubble Junky]

Have a look at Koh Lanta on the west coast of Thailand too - especially if you do a spring trip. April 13th is Thai New Year, and a great time to be around - one big water fight !