Yes, I prefer mirror diagonals--no chance of chromatic aberration.
As for large Dobs, the Obsession company (well-chosen name) makes at least 25-inchers, maybe even 30. Several of my friends own them. And they are made to be dismantled into smaller components, quickly and easily. You can transport at least a 20-inch in the back of a Subaru station wagon--they can be much more compact than a 16-inch Meade Dob.
I even once saw a set of 30-inch binoculars. Yes, you read right. A fellow had rigged two 30-inch Dob mirrors with some extra optics--a complex arrangement, but it worked.
As for getting a good scope for your money, there's no doubt that a 6-inch achromat qualifies.
You've made a reasonable choice.
But as for the contrast issue, I'll make a long post here in a couple of days. It's much, much more complex than just the "central obstruction" thing. And the resurgence of small refractors (I own one, by the way) has more to do with their ability to give very wide-field views with minimal vignetting and good off-axis correction than with the central obstruction. For deep-sky photography, the central obstruction is just a non-issue, but the vignetting from the baffling needed in SCTs is an issue. Also, the short focal length of the premium refractors makes them excellent wide-field camera lenses. I salivate over the ads for TeleVue and Takahashi premium refractors.