Donbas sure let them have it its full of russian separatists (not russian speaking ukrainians, theres a difference) but the Black Sea coast is a harder sell it'd cut off their only access to sea ports their only other borders are russian controlled areas or poland not exactly an international hub there. The real sticking point is Russia or rather Putin simply does not want Ukraine to have closer ties with the EU/NATO which is what this war is all about in the first place, Putin sees it as "spheres of influence" theres western spheres and russian spheres and as far as he's concerned Ukraine all or any part of it falls under Russia's sphere and will not accept anything less than strict neutrality at the very very least i.e. almost certainly no EU and absolutely no NATO membership and the ukrainian people almost certainly think otherwise, theres also the West's desire to see people have self determination i.e. its up to the Ukranian people who they choose to side with - and thats fundamentally at odds with Russia's authoritarian viewpoint that is the polar opposite. Hence the talks going nowhere at present
The last few weeks Zelensky has cooled on NATO membership (for obvious reasons). He went as far last week to say if NATO doesn't want us and clearly Russia doesn't then why would we want to join. Yesterday he effectively ruled it out live on TV. The EU topic is clearly however now what is wanted by Zelensky and you can guarantee the population want it to. But as you say I just don't see that as being acceptable to Putin. He also seems to have a serious issue with Ukraine having any sort of military - and that's an insane red line for any independent country. Ukraine has as right to defend itself under any international law just like any country. Why on earth would they accept no military when Russia (or even Belarus for that matter) could just come in when Putin gets out of bed the wrong side one day.
I agree about Donbas (at least the contested areas), I just don't see how they get to keep it. A deal where anyone who wishes to leave back to Ukraine proper is likely the only solution with some kind of buffer zone. They really should be able to keep the major cities and areas that the Putin backed fighters got nowhere near however. Crimea is gone not matter what.
Putin still has a huge amount of military power not deployed in Ukraine, but we have no idea the quality of it or the real manpower. Based on what we've seen there so far anything is possible. Also just how much of it is he prepared to lose? He still has his biggest borders and the rest of the country to defend. And there is the chance protests at home could get seriously worse and the Police need military assistance in his eyes. He can't fight everyone and win.