@PhilippaGeorgiou
So if we upped defence spending, would we be any less screwed if Russian missiles were incoming?
Not at all, but there is a coherent argument for vastly upping spending on conventional forces so we have the means to say, resist a Russian attempted invasion of Estonia, Latvia, or Lithuania, not only resist it, but eject the invader so thoroughly and comprehensively they struggle to recover from it. This is what would cause Putin to think more carefully about expansionism, and in doing so immediately reduce the risk that anything ever escalates to the point whereby there is a significant nuclear exchange between major powers.
albeit Putin appears to lack any sanity
I do not believe that Putin is mad, not in the way that is oft repeated. On the contrary, he's an extremely shrewd operator. He knows precisely how far he can push, and where, before western powers will object, he also plays the likes of Trump like a fiddle. He's persisted for such a length in Russia because he has played their Oligarchy well enough that they have almost a parasitical mutual interdependence.
Yes, Russia made a mess of the Ukrainian invasion, however, this is not indicative of Putin himself being "insane". It's a sign that some Soviet behaviours still persist, including obfuscation when truth would paint you in a bad light, and also endemic corruption right down to grass roots level in the military leaving it wholly unprepared for an invasion on such scale. Nobody was going to tell Putin that, because the entire thing is a massive chain of bullshit being filtered upwards, right from the very bottom to the very top.
Regardless, Russia is still, slowly but surely, defeating Ukraine by attrition, and nothing the West has said or done is preventing that from happening. Again, Putin holds all the cards here and is playing the West like a fiddle.
he would attack one NATO country when there is nothing at all to gain from such an attack?
You have to understand that the Western concept of "gain" is entirely different to that which exists in Russia. Putin is no different to any of his predecessors, in that he sees security as a question of space and distance. If, for argument's sake, he was able to snip off and annexe a Baltic State or two, it increases Russia's sense of security because it means NATO is further away from the heart of Russia, and it decreases NATO authority ever so slightly into the bargain as there is one or two fewer NATO members. You need to ask yourself, "is NATO Article 5 absolutely categorical and unequivocal?" The only time it has been invoked is in the aftermath of 9/11 when the US itself invoked it. Nobody hesitated to fall-in and assist the US, but say Estonia is attacked, do you seriously, honestly believe that the entirety of NATO is going to pitch itself into a conflict that could, very easily, turn into a nuclear holocaust, just for the sake of maintaining Estonian sovereignty?
The answer to that question is quite clearly "No", and Putin knows this. This is why it's fundamentally important to increase Conventional force to "don't even try it pal" levels, because short of that, Putin will try it, and once he starts, the outcome is inevitable. Estonia will be given up without so much as a whimper, because nobody is risking nuclear annihilation over Estonia.