You can use a UG/HG 7 speed freehub and run a 12t HG and a lockring. You don’t need remove any of the external threads.
The caveats with any Shimano freehub swap are the rear flange thickness, the dust seal diameter, and the obvious one, cones. The RM30 and Dura Ace 74XX are the odd spline/attach ones. And I believe the latest 11 speed.
You can cheat a little if you have a deep recess hub (thick flange) and you install an 8/9 speed thin flange. It won’t reduce it nearly as much as a 7 speed, maybe a couple mm’s, but you’ll need a spacer to make the large cog work.
John