1. No, it wouldn't. It would add complexity and another annoyance for the sake of complexity and being annoying.
2. War in Stellaris is already slow, tedious, and annoying. This idea would make that worse, especially for the attacker.
3. No, it would not. It would just lead to even more micro, massively benefit the defender, and make wars more frustrating and unfun.
Doomstacks are not a problem, never really have been. And any "fix" for them has made things worse. Because Doomstacks are a consequence, a consequence of winning, a consequence of how the war system itself works, how battles work. This is a bandaid aimed at annoying attackers and making the weakest aspect of Stellaris worse.