I'd say the most dangerous tool, if it exists, would be intel on the location and activities of high ranking russian officials and putin himself. We've seen people travel from ukraine into russia for sabotage (and vice versa). No reason these teams couldn't be equipped with guided missiles. VIP gets in a plane or a car and you blast him.

You make a compelling point, but that backs Russia to the wall of mutual nuclear annihilation. And push comes to shove, they will always care more about Ukraine than we will.

Weapon systems could be effective if they were mass produced in cost reduced versions. Europe should have been working on this since day one but they haven't been. With the supplies that are available, a significant impact to the russian economy or war machine could probably still be made by bombarding oil refineries and air fields with cruise missiles. This has been restricted by policy, not technology.

I doubt Ukraine can be supplied with, as well as maintain, any deep strike capabilities before the Russians eradicate it down to ineffective levels. No Western wunderwaffen has operated unscathed.

I also don't see weapons being the deciding factor, with the exception of artillery. Russia has something like a 5:1 artillery advantage. But it is logistically impossible to close the artillery gap without WWII Defense Act levels of production.