Unfortunately, some of the cheap altaz mounts I mentioned, seem to be - to put it bluntly - only a little better than toys...... due to their very limited ability to cope with significant tube weight and length.
I don't see why many of the small altazimuth mounts that are currently on sale have the telescope hanging a long way to one side of the azimuth axis. Its quite obvious that the tube ought to be right on top of the azimuth axis, in order for better balance: that way, a counterweight may not be necessary.
But thanks, Andrew, for the pointer to the Giro Mark II mount and the DM 4 and DM 6.
I note that on the internet, I recently found a mount designed for the larger and longer apo refractors called the Stellarvue M2