The movements due to temperature and construction errors make me think that axial movement of the 2ndry may as well be used for focusing. Linear bearings and ground shaft are easily available so I don't see making something like that up as much of a problem. Driving it is more of a problem in some ways. These bearings may have tiny amounts of play but that can be overcome if needed by using 2 spaced apart so that the max angular error is acceptable.
In principle if the main mirror is tilted in the usual way and centring added 2ndry adjustments of any sort aren't needed in theory providing it's assembly is reasonably aligned but who would build a fixed focus scope? One way or another the focus will shift in practice even if it's only down to temperature effects on the glass.
Good focusers are expensive.
An aspect I always think about and I feel that the 2ndry focuser is much easier to make. The downside of just adjusting the 2ndry for focusing though is eyepieces. I assume that their focal distance from their flanges varies but Celestron and Meade get away with that so it shouldn't matter too much. I sometimes wonder if people who use a mix of eyepieces on SCT's and find that some need a significant focus shift realise that they may be messing things up.
One things for sure a Cass is a challenge even to make the mirrors. That's my 1st step can I do that. Then I intend to try and make up a scope as I have outlined. If I have to go DK then I would still be inclined to do it the same way. Actually I suspect this area is why sct's move the main mirror with the added bonus that the scope will focus to shorter distances plus in their terms it's easier and cheaper to manufacture. Trying to do that at home would be more difficult than doing the same thing with the 2ndry.
The other point is where I would prefer the alignment controls to be - at the bottom of the tube where I can get at them while looking through it.
That sort of takes care of things needing tiny tweaks in practice. A sort of fall back if the idea doesn't work out as well as I would hope. I would also try and make the 2ndry so that it had zero wedge so that axial angle adjustments on the shaft that carries it wouldn't be needed. I doubt if that area is a problem really unless some one wants to use it as a spotting scope and needed a lot of focusing movement.