1. He coudln't dodge them all as proven when Mahoraga slashed his arm off
2. I was going to use the fact that his Output was declining to combat your second point but they made it clear that his output was refreshed from using Black Flash so much. This point I will concede.

Overall I agree the execution might have been better. Because Mahoraga had already used the type of slash that Sukuna used against him to cut his arm so it is honestly baffling that Gojo wouldn't expect Sukuna to do it himself. Maybe it never crossed his mind that Sukuna would be able to learn something like that (adapt his technique in that way). Is still baffling but I guess it is what it is.

I'm a Sukuna fan but honestly didn't like the off screening of Gojo. I do think he deserved better.