For deadlifts the stance really doesn’t really matter, just depends on flexibility of the hamstrings and ability to force thoracic extension. If you want to give it a go I suggest testing your 1RM or 3RM max for sumo and compare it to your deadlift, and use it if you feel stronger/more mobile.
For accessory I suggest lots of back and posterior chain work. The entire shoulder girdle must be retracted and stable through out the entire movement, and the posterior chain must be able to create the force needed to pull the bar. Basically do a crap ton of rows, pull ups, Face Pulls, Shrugs, as well as Back Extensions, RDLS, Rack Pulls for various sticking points, GHRs, and Hip Thrusts. Also if grip is a limiting factor do timed holds with a barbell or farmer’s carries with various implements such as Dumbbells, Kettlebells, Trap Bars, etc.
For OHP I would suggest back, shoulder, and tricep work. Back work should include face pulls and pull ups, shoulder work can include lateral raises and such, but upright rows and snatch grip/behind the neck presses can help a little more, and dips and iso work for triceps. Also make sure you add in Anti-Extension exercises such as Ab-Rollouts and Crunches to make sure you don’t hyperextended the low back, as well as work towards sufficient mobility of the lats.