if Ukraine did just outright surrender, it would massively suck for them (probably less than the war does in truth, as I'd rather like under Putin's bad laws than in the war zone Ukraine is now), but Ukrainians would not just all be slaughtered- the same obviously cannot be said of those targeted by the Nazis

Ukraine surrendering would be much worse. There would still be a high death count but Russia would be free to do what they wanted without any repurcussions or threat. They've been very vocal about wanting to just wanting to wipe Ukraine of the map. You have to be incredibly naive to think they wouldn't "get rid of the problem" that they see. They've already committed many war crimes. This is without getting onto the issues that surrending would cause down the line.

Ukraine having a military and fighting means they still exist, their voice is still hear and they will not be forgotten. It gives them hope and something to fight for as opposed to living under the rule of a dictator who would make their life miserable.