Huh. My players in my current Eberron campaign literally just finished their first model of a tank. They're pretty rich from finding a giant Siberys Dragonshard that they then sold to House Cannith for 5 million gold pieces, so they had the resources to create the tank. I won't describe the process of them doing so in this post, as it's not really important right now, but will elaborate further if people want to know more.
In my campaign, they made the armor of the tank out of an alloy of Mithral and Adamantine, so I gave it an AC of 22, but for a tankette like that, I would instead give it an AC of 19 (the same as steel). That also doesn't grant full cover like a full-sized tank would, so I would give it action stations similar to Infernal War Machines from Baldur's Gate: Descent into Avernus, giving 3/4ths cover to anyone operating one of its two action stations. One action station would be to command it to move (let's give it a speed of 30 or 40 feet), and the other would be to command the gun at the front (+8 to hit, 2d10 + 3 piercing damage on a hit).
Then, as that tankette would barely qualify for Large, I'd give it probably somewhere between 50 and 100 hit points.
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