Ukraine can't be admitted to NATO while there are any disputes at all ongoing. Putin knows this and continues to ensure disputes are ongoing. That's been his strategy for the past 8 years.
But this looks different. He doesn't want or need the regions that his invasion forces are "liberating." There's no significant economic or strategic benefit to those two regions.
That's why I believe this is just the next step to a complete invasion. Putin wants Ukraine. He wants its natural resources, he wants its people back within his domain. He doesn't want two small regions. He's trying to force Ukraine into responding so he can justify his next steps for complete takeover. But even if they don't respond, I don't believe he's going to stop after annexing these two "independent" states.
He wants Ukraine. All of it. And he believes it's his right to take it.