All

Going to 40:1 (less oil) will richen the jetting even more. Since there is less oil, that means there is more gas, per volume. That will cool combustion temps and cause more spooge. So, if you go to 40:1 you will need to lean out the jetting even more. I'd stay at 32:1 for now and lean the jetting out. One thing at a time so you don't get lost in the woods.

Yeah sparky will clog her up. And UNi's tend to be kinda thick with the 2 layers and all the oil and will richen the bike up a little too. You'll need to lean out the jetting accordingly as Bruce said. For tighter, single track type off road you generally want the bike leaner anyway since you're not doing much hard, wide open throttle and dont want the bike loading up and fouling plugs in the tight stuff. Having it nice and crisp will also help with that low end response you are looking for.

MX33 or similar. The sand tire would probably lose chunks when you hit the harder areas and those quartz ledges.

https://www.rb-designs.com/rb_designs_llc_011.htm Raced my 05 YZ125 in the AMA West Harescramble & D36 Cross Country as a B Senior and Super Senionr for many years with the RB Design mod carb. Google it... Theres plenty of favorable reviews. It's not stump pulling lowend but seat of the pant difference over a stock. I ran OEM and a FMF Factory Fatty with a Leo Vince spark arrestor, 13/50 gearing, 3.2 IMS tank and Utah Sportcycle bash plate. Applied clamps, Fastway /GPR stablizer with Fastway hand guards. For enduro I used a ICO Checkmate with remote thumb switch. Super easy bike to ride...

Man Bruce, Im sure he's aware its not the best tool for the job. Dont kill a mans good time, doing mods and enjoying the upgrades will add to the fun in my book not drain.

Here are a few tips and mods that you can do for more low-mid. It is a YZ125, so there are only a few things that can really be done to tune it for that, it still needs to be ridden harder. Stroke it, overbore it, or overbore and stroke it. A light FFW and all steel clutch plates. Retard the port timing by shaving off material on the cyl base, keep final squish clearance to 1mm...-.015" to .020" will help but it will take away a little top end overrev. Epoxy and profiled rear transfers so the ports appose...

The tire runs big, so that may not be a good option. I just looked and they don't make a 100 so it may not be a good choice. I didn't realize that when I suggested it. An AT81 or VE33 might be a better choice. You can probably get away with a 110 in either of those.

Jetting! Get that burn good in the 1/4 to 3/4 throttle position. Nice lower end pipe, then a flywheel weight, but not too much. Just my opinion.