I know it sounds a little counter-intuitive, but try placing a block of soft wood on top of that frozen stem then hitting that with the small sledge.

Idea being you may be able to free up the wedge nut by driving it ever so slightly deeper with the stem 'stead of relying on what's left of the draw bolt to stay rigid enough not to bend a little when struck rather than imparting the full force of your hammer to the wedge.

On my original MBGR's stem that draw bolt was about 6" long. When I started 'restoring' it back in '23 its wedge nut'd been 'frozen' for maybe 20 years or more. The draw bolt was fairly easily removed after I'd done the penetrant soak, but stem and wedge failed repeated attempts to remove by twisting the stem... why I'd abandoned further attempts way back then.

One caveat if you're willing to attempt this move: my MB's fork's steel, I didn't want to take a chance of doing it harm by leaving the front wheel mounted. So I pulled the wheel, fashioned a solid support to rest the fork part on where steering tube's mounted before giving frozen stem a couple of whacks with my three pounder. That steerer's not plugged so I could drive the wedge nut out from the bottom; here's what it looked like after all those years inside: