>Well shit. So if the lore is set in 21st century then Bolo tonks are not possible to develop?
We are already living in the 21th century, and it's perfectly possible to build a gigantic tank. It just makes no sense, because a ship or a bunker would be much superior for anything you could use it.
>other hypothetical weapons
There is nothing hypothetical about cannons, tank, or nuclear shells.
>gauss cannon, rail guns
Those are used to accelerate a projectile in a manner that is much more complicated than simply burning some gunpowder in a tube, and making anything other than simple APFSDS projectiles is harder if you are also trying to electrocute the shell in the process.
Closest things we have right now are lasers, although there was that one time when a lab made a short-ranged plasma gun (look up Project MARAUDER). But with most scientists being busy chasing the mythical dark quantum powenergy I don't think we will see death rays in the coming decades. As such, speculating about particle accelerators is a futile attempt.
>Are those weapons practicable enough to with a increase in scale to be mounted in a tonk as even more effective means to eliminate a (very-) large monster?
But why do you need a tank, when the only thing that matters in this context is the projectile? Your best bet would be to take one of those stupid fast Russian anti-ship missiles (there are a few varieties of them), put a HEAT or a nuclear warhead on it, and see how much it hurts our monster. All you need is a(n admittedly big and purpose-built) truck with no armour whatsoever. Tanks are meant to go to the enemy and engage them in line-of-sight battles. Doing that against Godzilla is suicidal.