If your lumbar rounds the moment you pull, it's not weak abs, it's a setup and bracing problem. Three things, in order:
1. Bar position. The bar should be over the middle of your foot, ~1 inch from your shin. Most rounding starts because guys set up with the bar too far away and have to chase it with their hips up first.
2. Lat tension before the pull. Before you grip, pull your shoulders back and "bend the bar around your shins" - this engages your lats and locks your upper back. Without it your thoracic spine collapses and your lumbar tries to compensate.
3. Brace, then pull slack. Big breath into your stomach (not chest), brace like you're about to take a punch, then pull the slack out of the bar, you should hear a soft click. Only THEN drive the floor away.
If you still round at heavier weights, drop 20% and do 5x3 from a 2-inch deficit for 3 weeks. Deficit pulls force a better starting position and teach the back to stay neutral under load.
If your back rounds at 50% of your max, the issue is technique, not strength. Fix that first; don't keep grinding reps that ingrain the pattern.