Do your normally pull with a rounded upper back? All the videos of yours that I remember seeing were from the front. If you pull rounded then it’s normal to be weak at lockout. If you don’t normally pull rounded and this was a technical breakdown then you need to get your upper back and lats stronger. Either way, adding bands and chains can help your lockout.
Fat grips are way too thick to work for deadlifts, your back off sets will be limited by grip. I own a pair and I was disappointed the first time I tried deadlifting with them, I don’t think I could pull 315 with them. Do separate grip work instead. Static holds for time, heavy static holds until your grip gives out. I have been doing shrugs with thumbs over the bar, I got that from Ed Coan’s book. I pull hook grip and I don’t have any grip problems (I just don’t want to have any) but everyone that I have heard discuss grip training has said to use a double overhand grip, which would make sense because it is more challenging. Another thing, hold the last rep of every work set for 5 seconds, that alone should help.
You’re inches away from a 605 pull, good work!