Russia-Ukraine War (Threadbanned in op)

The only red line for Putin is a nuclear attack on Russia because he'd have nothing else left to fear. He's a bully and like all bullies is ultimately a coward. He'll rant and threaten but ultimately he only has one ace up his sleeve and using that ace is signing his own death warrant.

no one expects Crimea to fall ‘immediately’. If Crimea ever comes back to UKr then the return to it has already started with all those strikes on big/small boats /ships, etc , etc, etc, etc.. the Russians do not currently know/will never know when , where ,with what and with what intensity the UKr will strike. There will then come a time when Putin has no choice but to withdraw Because of too many losses, too costly, an about turn by his replacement, domestic coup changes things, etc, etc, etc.

The constant drip by drip bombardment cannot be ignored by Putin as insignificant

If he launches a nuke, it'll be from Belarus or one of his proxies.

Tbh I don't think taking back Crimea by occupation is necessary. They just have to permanently deny it to Russia by making it untenable for them. That's happening by totalling the black sea fleet in sevastopol and threatening the Kerch bridge. They do need to get the eastern land corridor though now that Russia has built a dratted railway to Crimea. Destroying the bridge last year would have been fab, this year not so much. The allies are always behind the curve in this war.

The land corridor isn't really that great an improvement for the Russians, though. If the Kerch Bridge is gone and their ferries are being blown up in port, they're still reliant on a single supply line, and one that's actually closer to the front line for much of its length than the Kerch rail connection. And then it has to cross the peninsula's choke-point to be of any use to whatever Russian troops/bases/people are still in Crimea.

While in theory a rail line can be repaired quickly, the materials and equipment still have to brought to the location and remain there for as long as the job takes. The Russians can't provide air defence along the whole stretch from the border to Crimea, and it only requires a single track to be buckled to make it unusable. So with the munitions they have at the moment, and moreso if they could whizz an F16 over the top, it'd be relatively easy for Ukraine to blow up different sections every few days and demonstrate to Putin that the whole project was a waste of time and resources.

There's no point in doing that while the Kerch Bridge is still passable and there continues to be a Russian presence in Crimea that's still fairly functional. I reckon the Ukrainians will slowly push the military infrastructure there to breaking point, then wallop them in such a way that it's impossible for sufficient reinforcements to be delivered from "mainland Russia". At that stage, they won't need to fully re-take the territory - just put up Ukrainian flags in various places and make sure that anyone trying to take them down meets a friendly grenade carrying drone (they did something similar with the Terrikon waste heap).

How is Crimea gonna fall? … they could only make small dent during last years summer offensive in the south

Speaking of Crimea, getting hot again...

Those train carried tankers sure do look like pretty fireworks!