Try to focus on things within your control. Make sure you get your necessary sleep (7-9 hours) and eat well. Perhaps consider lifting before work, even if that means waking up at 4-5am. Maybe try to get a main session on a day off and another session or two on the days you work before work.
I get better sleep on AAKG supplement(4-5g). I also take melatonin.
Work to reduce stress as much as you can. There are also some supplements you can try like ashwagandha and bocopa monnieri which can help.
I have worked six years in retail and know a little bit of what you have to go through. I'd strongly consider making physical requirements a part of the hiring requirements for the positions(since as a manager you probably have direct input). I would also think about which physical job requirements the other staff can handle. Most women can lift 30 lb. Multiple women can lift heavier objects as a team. As a manager your role is to lead, supervise, manage, train/coach(e.g. proper mechanics to lift heavy objects as a team), etc. While doing the same tasks every so often or with the staff is a morale booster no doubt, if you do it in place of the subordinates then that's not good either.
Also make sure you have good pair of shoes for work. I'd buy separate good insoles. Sometimes I spent as much on separate insoles as I did on the shoes and they were well worth it. It will save your feet/knees/back/etc.
