Could RC planes like this be armed and used as makeshift combat UAVs?

Without having to pull it up, yes. Small caliber firearms, grenades. They don’t have to be stand-off weapons to get in close and get the job done.

They absolutely can be used. They do not need to be armed with an RPG launcher. Another high explosive with a shaped charge can be used. They can be fitted with live video so they can be guided in to the target.
The usage depends upon the distance to the target and the target’s hardness.
If this is to be a one way trip you can trade off a lot of fuel mass for more “energetic” materials.

It is just that the thought of brutal russian invaders getting unexpectedly strafed by an oversized toy Panavia Tornado makes me have a proper giggle.

Are you thinking of this?
Drone

Disclaimer. The targets he is using are most likely filled with tanfghnerfghite (remove the fgh) a readily available 2 part bang powder that is used for “display purposes”. His device is propelling a mass with just enough energy to excite the stuff.

His unit would be immediately knocked from the sky before he got close enough to do anything. Still, it is impressive work for a hobbyist.

Looks vaguely sexual.

Maybe program it to deliver lethal shocks when they start humping it.