I'm using a quick change cutoff holder. I have two lathes, a large one that uses the 400 series QCTP, and a smaller one that uses the 100 series QCTP.
I used to have problems with cut off operations on the small lathe. It is a 10x24 Jet brand and a light duty bench lathe (about 350lbs). It would dance on the floor when it went into harmonic oscillations. The fix was a piece of 2 inch thick steel that I purchased and bolted the lathe to that. This made it rigid enough that it fixed all the issues with how wimpy this lathe was.
I did mount the piece of hot rolled steel on my vertical mill and faced it off, then drilled and tapped the 6 bolt locations of the cast iron lathe base. I used shims to level the lathe so it turns true.
Cutoff operations seem to be the number one issue with home machinists. And it really appears to be a light duty lathe issue, that really can only be fixed by increasing the mass and lathe bed rigidity.
I like your design of a blade holder, simple. It does require grinding a back relief to the cutting edge.
The down side of the QCTP holders, they hold the blade at the relief angle, so if you change the stick out from the holder, you have to adjust the height. Not a big deal, but it slows down the quick in QCTP when you are doing a deeper or shallower cutoff. And I only shorten the stick out when doing a harder material like stainless.