Zenbot/Gecko/LinuxCNC?

I think your assessment of Mac3 is fair. Its clearly had its day and I'm not using it for the same reason.
I can't comment about UCNC but I do see a lot of threads on other forums recommending using it. I did not know it existed when I started. Hewre's one last shot at convincing you but ultimately, you are making the decision.

Installing a preempt_rt image and Linuxcnc is simple. You just need to grab one of these ISO's
www.linuxcnc.org/testing-stretch-rtpreempt/

Because you have an existing machine, configuration is probably not that bad. I think the other thing in your favour is that you have a conventional milling machine and therefore won't need to push the envelope when building your machine like I had to with a plasma machine.
I've not used it but there is a Mach3 converter in stepconf that will build an equivalent config for a parallel port that takes the XML file as input.
Its not going to work for a Mesa based machine but it might be very useful as a reference files.

With Mesa you need to use pncconf to create your initial config but it is also very comprehensive. pncconf has recently been enhanced to give better gantry support (but those features might require using version 2.8 - which you can load from the Synaptics package manager if you follow the docs).

Whilst its very different to Mach3, the Gmoccapy interface is very nice to use and I doubt you'd need to modify it. The new version of pncconf allows your to select the interface from a drop down but its only a 1 line ini file change later.

Before you make your final decision, I'd suggest you at least install one of those ISOs and have a play with some of the many included simulators (or SIMs). Any hardware would suffice for the SIMs as you are not driving real hardware in real time. See if you can generate a config from you mach XML and review the generated INI and HAL files.

Excellent. I'll do just that. I did find the Mesa configuration wizard but it said is was beta and I shouldn't count on it. (Out of date documentation?).

Thanks for info on the iso with the RT kernel already installed. That was another stumbling block.

Finally, I'll look at Cmoccapy. I'm not too picky, but I need to be able to do automatic Z touch off at least based on a probe. Does it do that?

OK so I lied, there is one more finally. I'd like to buy an MPG that is pretty much plug and play.

Just looked at Gmoccapy real quick. It looks like it does do Z autozero but not the way I want to do it. I want to put a conductive plate of known thickness on top of my stock. The router is connected to the other conductor so the bit is too. I might eventually go with something with a separate tool change block, but not yet. I certainly want both options.

Gmocappy supports a series of macro buttons. You could program one of those to do your touchoff using gcode. For the record, I believe the G38.x probe functions in Linuxcnc are superior to those on Mach. You do it something like this (from my plasma setup)
You'd just change the G92 to set the height you want.

The shoot out between LinuxCNC and UCCNC continues, with attempted test drives of either option

LinuxCNC

I downloaded the iso previously referenced iso, put it on a USB flash drive, and booted. There was list of different versions to try and boot. I tried all four RT kernel versions, because until the last one, none of them booted properly. The one that seemed to work was second in the list.

This version of Linux doesn't seem to support tap on my touchpad, within a couple minutes I had to go get an actual mouse to relieve frustation. Not a show stopper, but Ubuntu doesn't have this problem.

Fired up LinuxCNC, tried various sims. I was quite surprised that I couldn't easily find a jog window in most of them, this seems pretty basic. And the standard jog keyboard commands from Mach3 and UCCNC don't seem to work. Didn't find any easy way to figure out what they might be. So there isn't much I can do with the sim.

The Gmoccapy sim crashed, complaining about a missing module gst which is apparently needed within some python module to play sounds.

The problem with boot and the mouse means that I would want to verify that the ethernet port actually works before ordering a Mesa board. I would have to figure out how to do that.

On a positive note, I did get the sense that if Gmoccappy weren't broken, it would have a decent look on the screen, probably better than the UCCNC screens

UCCNC

Since this is a Windows program, the install is seemless, and the program came right up in demo mode. The jog keys worked and everything seems functional. No hassles.

But I really don't like the default screen. I would definitely want to buy another screenset which I could get for $25 here:

www.thecncwoodworker.com/2017.html

So that would bring the total cost of UCCNC to $159 + $67 + $25 = $250, vs. $89 for LinuxCNC

That's a little annoying, but I do have to consider my time.

I'm not giving up on LinuxCNC quite yet, maybe I'll get some help here. But so far my CNC mentor's words are ringing my ears.

OK, cool. That's fine keyboard jog is plenty, and that got it working. But now how to do I zero the X axis (without issuing gcode commands manually)? I don't see zero buttons for the axes (or keyboard commands).

Also, should someone know that the Gmoccapy sim seems to be broken in the version I was able to boot? And is it normal that three out of the four options on the ISO won't boot successfully on a laptop?