Just got done reading the entire post -- great build and thanks for the inspiration. One of these days I'll get my own SAS done...
About the tire balance thing... Both of my kiddos work at a local shop -- in fact, my oldest just bought into a shop as part owner -- and from their experience in mounting 20 + sets of big x huge off-road tires a week, the junk inside the tires for balance JUST DOES NOT WORK.
Go somewhere that actually understands off-road tires, and has the equipment to deal with them and get them balanced with external weights.
That will solve about 90% of your 50 mph issue.
We've cleaned so much gunk out of tires from guys reading posts on-line that say that the junk is supposed to balance the tires that we collect it in 5 gallon buckets -- from golf balls, to bb's, to air shot, to lead shot, to ball bearings, to tennis balls, to whatever the mind can get around, we've taken it out of tires. Most are shredded on the inside from all the friction -- NONE ever ran down the road without humping all over the place. All (that survived being filled with junk) ran nicely after a decent balance.
We generally use stick-on weights as near the center hub as is possible. If brakes or something are problematic, we use the regular clamp on weights, surrounded by a bead of silicone to make sure they stay there.