The main reasoning was to use existing gun, in hopes of not making the design too fucking hard (new chassis+new engine+new gun?). We do not need new gun. We have enough good gun. Just pick one.
90mm AA/AT gun from Moskurg, Arstotzkan 80mm artillery or the Moskurg 100mm artillery gun.
The 90mm AA/AT gun equivalent would be the 88mm KwK 36 from Tiger and other famous German tanks, which originally was an
heavy anti-aircraft gun or the 85mm D-5 of Soviet T-34-85 fame, which was originally
also an anti-aircraft gun, or the American
90mm anti-aircraft gun, mounted on Pershings and M36s.
The rough equivalents for 80mm artillery and 100mm artillery would be
Soviet 122mm artillery guns, which ultimately ended as main gun on IS-2, the
100mm tank gun which originally was an naval cannon, or whatever, since I am too lazy to look further. The point stands - we can reuse existing guns, we just need to equip them with proper ammo and mount them on self-propelled chassis, be it a turreted tank, or casemate tank destroyer. The quivalent for the 150mm naval cannon would be of course the 152mm ISU-152 cannon mentioned before.
As reusing goes, we could also hope to reuse the Haast (or whatever, as long as it's not a fucking turbine please) engine,
as using aircraft engines in tanks was also a thing.This way we take out the new gun and new engine out of the equation, reducing difficulty, which leaves us only with chassis. If we wanted to go even lower, we could use the AS-33 with new gun in casemate, effectively reducing the new tank to what amounts to really hard revision.