Oooo...
Decisions, decisions!
If you are happy with your dob, you could upgrade to a larger version with 'go-to' capability and then add an equatorial platform for AP as and when.
This's probably the easiest route, but although portable, not very convenient for moving around.
If you are planning on doing serious AP, I think you shoud spend the bulk of your budget on a decent Equatorial Mount. Choices are a bit limited in this price range, but a Skywatcher EQ5/EQ6 is a good start.
They're not perfect, but all their foibles are well documented and they don't cost a fortune (1500ish new - 1000ish used).
Meade has a new dual-format mount coming in the new-year that will work in alt/az (up-down,around) mode ar as a GEM.
It looks good, but I don't think anyone's actually got their hands on one yet.
To this, you could add a larger (8"-10") Newt, an SCT of about the same aperture, but portability comes into it again.
For a good balance of features the, tried and true refractoror is hard to beat. There are LOADS of high-quality, relatively inexpensive refractors.
Of all those out there, my favourites are:
The Sharpstar (Astro-Tech) 106mm f5.5 APO (about $1500). A really stunning performer. TMM, the best 4" refractor available under 4K.
The venerable Skywatcher ED80 (about $700)
The William Optics GT81 (about $1300)
The Orion / TS / Ikharos / APM 80mm CF Triplets. (about $1000)
All the above are also excellent for visual use.
Then there are literally hundreds more choices if you are prepared to buy used.
If you are having real trouble, you can always send me the dosh and I'll sort it out for you!
Happy hunting.
Peter