You could shitout thousand of drone UGV with decent steel armour based on designs made in the cold war era and theyd be good enough to kill a modern tank, have the armor to survive small arms and youd have so many the enemy wouldnt have enough ATGMs

>>64000659 (OP)
The gun is the most expensive part of nearly any tank, especially a modern MBT. They wouldn't be that much cheaper than an existing design, even if you skimped on armor.

>>64000702
Theyd cost less than a modern tank, so you could easily do a wolfpack ganging up on them,
You could suplement shittier optics with drone optics and have them carry surveillance and FPV drones.
After that just shoot a bunch of times achieving Mission kill because modern tanks cant work without their opticsand the crew will be concused, maybe just go with squash head rounds.
After that out a regular one in the rear of the tank so they cant recover it.