Which hollow wall boxes are the right ones?

What you can still consider at special spots are multimedia boxes, like the Kaiser 1068, where you can also fit a bit of electronics inside.

How do you get the thick thing through the opening for the hollow wall box?

Admittedly, I walled them in, in drywall I only used standard boxes. But I understood the shape in such a way that it is just shaped so that it barely fits through with sufficient force.

Kaiser 1068 would not be suitable - for drywall partitions the option Kaiser 9268-94 should be installed - the tunnel is flexible there, and is pressed in during installation, then stretched again inside. Works fairly well (as long as no beam is in the way, see above), but the tunnel is not very durable either - it should not be removed and reinstalled too often, etc.

Thanks to Harakiri for mentioning the correct can, mine is only for flush mounting. The tunnel also seems smaller to me - on the one hand understandable and better than nothing, on the other hand my SBC with all the cable stuff probably wouldn't have fit in. So test sitting beforehand!

Definitely - it is definitely sporty. I had quite compact KNX actuators installed, but even for that it was relatively tight (at least including Wago terminals). If any manufacturer offers larger models (I think OBO has something similar), you should definitely compare and, if necessary, switch.