I recently had (for the first time in years) a stuck collet nut on my 2.2kW spindle (similar to the HDM’s) and I was able to salvage it. It turns out that in my case brute force was the answer, see this thread for the horrible bespoke tool I used:
I thought I would permanently damage the collet thread, but it turns out that after a few more operations of the collet nut on the thread, it is almost as smooth as it ever was.
Just mentioning this in case it helps if you are going to get a new spindle anyway