Does the gimbal point between the center bolt and the holder need to be where it is (seems fiddly and inaccurate)
The top figure is the way I design secondary holders, the one at the bottom is the "typical" way to hold secondaries.
In my design, the center of least movement is almost coincident with the optical axis, so the tilt and tip are not moving the secondary laterally (or longitudinally). In the typical design the center of least movement is almost at the top of the secondary mirror, so, tilt and tip adjustments move the secondary off the optical axis. This is more important as optical speed goes up.
Notice, also, that the point where forces are applied to the secondary mirror have:: a) almost no distance for bending moments to transpire, b) impinge directly on the silicone glue holding the secondary to the holder. Compare this to the typical secondary design and we see fairly large distances where bending moments can show up, and large cantilevering distance--leading to being vibration prone.
1) tilting the secondary around it s minor axis, locating the actual axis center point of rotation in height with the optical surface
You want the secondary tilt/tip to articulate the secondary as close to the optical axis as possible.
2) rotating it around the main mirror optical axis
3) moving it "up" and "down" along the main mirror optical axis
These are what my design avoids.