Anti-Tank Gun may as well be one of the most pointless units in the game, on paper it may look quite useful, 3700 industry cost is less than the 7200 of a medium tank. As an heavy weapon it can dig in for a +10 defence against ranged attack for a defence of 63 which is higher than the 61 combat strength than medium tank and the armor piercing ability give +8 combat strength bringing up the Anti-Tank Gun strength to 61 when fire upon armored units.

However in practice the Anti-Tank Gun have many issues:

• The dig in bonus can be removed by artillery or helicopters, units you should spam anyway, you also have to choose between fire or dig in
• Can't move and fire and depend on line of sight
• The armor piercing ability only increase the offensive power, it don't protect against return fire
• Anti tank gun have very low base strength for a contemporary unit at 53, without dig in it is going to be destroyed in very few hits
• The anti tank gun offensive power even with armor piercing is too low, at 61 it is just at the same strength level as a medium tank and inferior to armored personal carrier, main battle tanks is even worse. The only units it would really do well against is ships as it have good range and many ships in contemporary is armored thus the anti tank gun get the bonus against them, also ships have quite low combat strength which also help alot.

• Offensive power the difference is 0
• Defensive power, with dug in 2, without -8
• Cost half as much as medium tank

• Offensive power the difference is -2
• Defensive power, with dug in 0, without -10, much worse as the APC also suppress
• Cost the same as APC

• Offensive power the difference is -8
• Defensive power, with dug in -6, without -16
• Main Battle Tank cost 4 times as much as anti tank guns

In terms of cost, this may be the best matchup for anti tank gun given the massive cost of main battle tank, but that follow the flawed view of seeing units indivudally and assuming the battle will be 4 vs 1. In reality battles will be much bigger and use multiple units. The main battle tank mobility, high combat strength and having armor percing is far more valuable than can be showcased in a 1 vs 4 against anti tank gun.

Combined with artillery to remove the dig in, an main battle tank will one shot anti tank guns and in a 1 vs 1 duel it will win no matter what and if it become damaged it can try to retreat with its excellent mobility, it is a far superior unit to anti tank gun, far more survivable, mobile and more useful.

Basically the anti tank guns really can't beat tanks, except in a very generous condition in which they somehow get to dig in, the enemy don't use combined arms and the mobility of tanks. APC completely make anti tank guns obsolete. Also this don't even talk about how lackluster anti tank guns is against non armored targets, its lack of flexibility and its poor performance even against the stuff it is supposed to be good at put it pretty much in the useless category of units, the only thing it really do well against is some naval unit of all things.

Changes I think should be done:

• Increase the armor piercing ability to +12, this make anti tank gun more effective, especially against medium tanks and APC without making it against stuff it is not supposed to do well against such as rifles.
• Increase APC cost to atleast 7200. This unit is simply too cheap, the Australian emblematic replacement cost 7200.

To be honest, anti-tank guns were mostly a thing during World War 2. Rockets made them obsolete very fast if I remember correctly. They made an emblematic thing of WW2 into a unit, but really, it's perhaps an annecdote regarding the span of life of humankind, and even the XXth century.

The possibility to upgrade anti-tank guns into some other new kind of artillery would be logical. Nowadays, a tank can be destroyed with a simple rocket launcher. (In fact, during my studies (WW2 again), I found out that tanks were often hipped up a lot in research and pop culture where, in fact, combined arms is really the only way they have to shine, and even then, they are giants with a lot of vulnerabilities.)

All of this pushed aside, your analysis is again very well tought out and your solutions sound like bare minimum.