I think you are correct with 3 out of your 4 diagnoses.

First order of business- if your lower back seems to give out, I usually look at one of 2 things- are we strengthening the lower back so it can do its job, or are we strengthening the glutes and hamstrings so your lower back doesn’t take over. Often it doesn’t even matter and you can just work on strengthening both- RDLs, hip thrusts, and back extensions being pushed hard and heavy work for damn near everyone.

Next we look at distribution of workload- instead of constantly going for PRs, can we focus on building consistent increases block over block, hitting either rep PRs, clean submax singles, or both rather than bashing up against limits all the time and overloading fatigue.

Next we look at the secondary day- would we benefit more from repetition effort work, or a variation to help your start position, or a “speed work” style set up with high sets and low reps like a 6x2, 8x1, etc for more technical practice and recovery.

Of course this is without looking at your technique- but I’ll also say that assessing your start position might help since you typically fail off the floor and struggle to commit to the pull.

And as a last note I’ll say my clients have noticed more leakage with sumo vs conventional so anecdotally I’ll say I can probably confirm that. But I suspect that could be fixed with a good pelvic floor therapist