As discussed simplest way is to get bigger and add more muscle in the back and you’ll get stronger. Even getting the prime movers (quads, hams and glutes) bigger/stronger will assist because you’re back will be less prone to rounding out as a compensatory mechanism to put your hips/glutes into a better position to break the floor. A caloric surplus and driving up volume in relevant exercises will get you right.
Vaguely recalling your training log I don’t think you’ve been doing much in the way of rows, shrugs or other upper back accessories (correct me if I’m mistaken). Sometimes there’s only so far you can take the competition lift especially when it’s reduced ROM such as the sumo i.e. geared more for efficiency than getting in more work.
A conventional/hip hinge variation with greater ROM and time under tension will be more effective in building muscle vs sumo. That’s a lot of bang for your buck already muscle group wise and there’s plenty of variety in lower and upper back exercises that you can perform for high rep/volume/frequency to build up those area also.
One consideration I’m undecided on is loading.
You may want to consider doing some heavier stuff e.g. heavy shrugs, rows and even cheat rows are challenging your back with heavy loads tho it’ll be quite taxing. Even just standing up holding 300kg is challenging your back (not recommending this just using it as an example). Going heavy might be more useful then doing exclusively chest supported rows with light weight nomsayin (tho that should still make up part of your routine). Hell front squats too even tho I hate them personally and strongman carries could work. Lots of options pre much but consider specificity of joint angles and loading.
The second approach is related to cuing, technique and assistance exercises that improve the movement/technique.
Cuing wise I think patience off the floor is a big one and a favourite of many lifters e.g. Bryce Krawczyk’s. Many lifters sacrifice a good position for an easier start but it doesn’t matter how fast the bar blasts off the ground if you positioning is too fucked up to lock out properly.
Other cues have been covered by other users e.g. pulling the tension of the bar and yourself as a system and various cues to engage the lats and back to increase spinal stability
There are movements that emphasize and let you spend more time in problematic parts of the ROM and challenge your ability to maintain good positions e.g. flat/neutral back can work well for many lifters. At worst it’s more work/time under tension for hypertrophy purposes.
Pauses are good shit but apparently may not be enough on it’s own or as you’ve been doing them. I might have been the person who suggested this in the first place but it’s possible that for all the cuing and assistance work that we do the legs are outpacing the back in strength: an “upper body” v “lower body” weakness in the DL if you will. So we need to structure training to challenge the back more.
Pausing at higher percentages is an option and so is including tempo work on the eccentric part of the deadlift because IIRC you usually skip it entirely. That’s a lot of work and time under tension in keeping your back straight under load and positions specific to the deadlift.
IMO a combination of these two approaches is called for if you like concurrenty kind of periodisation or at least as per phase potentiation with a hypertrophy block first then a strength block.
