Trump was absolutely correct that Zelensky doesn't have any cards, but as of the fall of Kursk, neither has Trump. It's not his fault, but it is what it is.
- Weapons? Useless.
- Sanctions? Not very effective.
- Direct military reinforcement? Out of the question.
- Kursk? Fallen.
The only thing Putin has to lose is more of his men in the course of the war, but I'd guess the majority of Russian soldiers and civilians believe the fight is worth it at this point. Russia will whoop on Ukraine some more and eventually there won't be anything to negotiate with.
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.
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.
It doesn't look like it's gonna go that way though. They'll carve up ukraine, americans will shrug and move onto the next thing, euros will seethe. If euros care so much they should try working at least 40 hours per week.
Assassination is absolutely out of the question. Because WW3.
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.
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.