Supporting Ukraine is still more or less voluntary....
Not really, this is the NATO statement regarding Ukraine membership:
We fully support Ukraine's right to choose its own security arrangements. Ukraine's future is in NATO. We reaffirm the commitment we made at the 2008 Summit in Bucharest that Ukraine will become a member of NATO, and today we recognise that Ukraine's path to full Euro-Atlantic integration has moved beyond the need for the Membership Action Plan.From:nato.int/cps/en/natohq/official_texts_217320.htm
Zelensky wants some kind of more guaranteed membership confirmed, even if it isn't immediate, while NATO has kind of kept the same stance it has already had. That NATO welcomes Ukraine joining but it needs to follow a process and NATO can't guarantee future membership at this time since it is condition based.
I understand why Zelensky would want a guarantee at this point though. He is concerned about Ukrainian NATO membership being a bargaining chip in the eventual peace agreement, while he views NATO membership as the only real solid guarantee against future Russian aggression.
I don't think there was ever a realistic chance of Ukraine being granted confirmed membership now, even if it were time delayed until the conclusion of the war. I expect Ukraine will join NATO quickly following the end of the war, though.
If it were up to me I would grant Ukraine membership now and have NATO immediately join the war under article 5 and begin strikes on Russian forces and infrastructure until Russia leaves all Ukrainian territory (wouldn't take more than two weeks, maybe even just 3 days), but that just isn't realistic given how many countries in NATO don't want to get involved in a direct war with Russia.