Looks nice from the datasheet! Running a few calculations based of times I think you don't want to push the speed to much, I got 200Khz. It is not worthy it says it's optimized for hard switching. A drsstc soft switches so you might not get as much benefit from that. I've never tried the transistor so I cannot say for sure. Still seems real nice!

I was wondering though would a small 120VAC VFD work as the incoming source?
Bob

Why the heck not, if you've got such a thing.   I'd beware of accidentally setting the frequency too low.  120 volts DC might immediately blow up your sync scope (or any AC motor driven by the VFD).  For protection you could put a fuse or a capacitor in series with the variable frequency source.

Square wave inverter sounds like a good place to start.   Aren't most of those "modified square wave inverters", with some intervals of low impedance 0 volts in between the +V and -V phases?  That greatly reduces harmonic distortion without adding much complexity to the output bridge.  Even if the inverter frequency is crystal controlled, I bet it differs from your grid frequency enough that your sync scope pointer will move.

I have begun a habit of looking at everything with the thermal camera so I can catch problems before they start a fire lol.

Yes, me too.  A very useful tool.  Even more useful with a closeup lens.  ZnSe lenses are available at reasonable cost (~$20) intended for CO2 laser output focusing.  I find 100mm focal length most useful.
After a while you will get used to ignoring resistor hot spots.

Bob, your OP says you want a knob to control the continuous rotation of pointer, adjustable from running a little bit slow to a little bit fast.
I think that rules out a "Freau phase adjuster", which would need to have 360 degrees of adjustment range, and a control knob turning continuously to make Sync Scope pointer turn continuously.   As I understand it, you really do need a source of 120 volts AC with frequency that can be dialed between, say, 58 and 62 Hz.  No big deal to get a digital square wave with frequency range like that, e.g. with a 555 timer circuit.   The challenge is to get the signal amplified to appropriate voltage and power for one input of your Sync Scope.

How about starting with an inexpensive DC-to-AC inverter, as used to run small AC appliances from battery power in vehicles?   Depending on accuracy of the inverter's built-in frequency reference (before modification), your scope might rotate in fast or slow direction to begin with.   These days some of those have battery built in.  Another way to get fast or slow rotation is with throttle regulation of an engine-powered generator.

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So, the two big resistors near the three output terminals start creeping over 60 degrees if I’m putting in more than 16v, (no fan yet obviously) does that seem wrong to anyone? The rest of the setup looks much cooler from my thermal camera. At 24v in the resistors kept heating up into the high 60s before I turned power off.

Power resistors like these are designed to run hot.  200C surface temperature is common.  Only real concern is their heat warming adjacent capacitor.  A fan will help with that.

200! wow, ok good to know. I have begun a habit of looking at everything with the thermal camera so I can catch problems before they start a fire lol.