How is this thing going to move through plants, mud, trenches, etc…?

It may not need to if it just stays on roads. That, after all, is the primary method for vehicles to travel, which are its intended targets.

Also, why build a ground kamikaze, if we already have quadrocopters?

I can think of two reasons. Cost-per-kg-of-explosive delivered is much lower with this than quadcoopters, and loiter time. You could drive this thing with its huge explosive to a nice hiding spot miles away and leave it parked there for days. Your quadcopter loiter times is usually measured in minutes.

So use your quadcopters for observation, then drive your closest ground based drone to the approaching tank or vehicle.

Traversing fields and roads to park on observed Russian tracks is the game.

Aerial drives see the route Russians use, (and therefore think is safe), then this drives and mines it overnight

Weight isn't as much of a consideration for a ground based unit, so you can have a bigger boom. I don't know if that's really required, but it could be useful for clearing entrenched positions.

For armor, the least armored location is usually (pretty much always, maybe with some weird exception) on the bottom. If you can drive under it, especially with a bigger explosive, even the most heavily armored vehicle is toast.

That said, you do need to reach it which will be harder on the ground obviously. It will be interesting to see how useful it'll be. If it's cheaper or similarly priced to an airial drone, I'm sure it'll come in handy. It'd be a cool bonus if it can deploy its payload and leave before detonating it and be recovered, but I'm assuming that isn't an option.

Idk about this specifically but a decent monster truck RC car would probably handle some terrain beyond asphalt. Maybe. I think you would need a larger scale to stand a better chance of success.

But yeah using quadcopters probably makes more sense.