Hot water issue

First time posting so bear with me. I had a tenant who complained about their hot water running out after 8-10 minutes. After running continuity test on the elements and making sure the thermostats were working, my next thought was the dip tube. I replaced the old dip tube which was non-existent when I removed the cold inlet nipple. The water kept coming out brown/orange from rust/sediment build up in the tank after replacing the dip tube and then the relief valve started dripping. The water heater was 13 years old so I decided to replace it. The old unit was an electric 38 gal. low boy.

I purchased a new unit at Lowes but they only had 30 gal capacity low boy units that will fit in the closet without having to remove the door casing. I have changed out roughly 10 water heaters in the past 2 years without any issues. About a week after installing the new 30 gal unit, the tenant informs me that they are still only getting about 10 minutes of hot water. Thinking I may have gotten a bad unit, I ran continuity tests on the elements/thermostats. Everything checked out. The unit says it should produce 43 gallons of hot water the first hour which should equate to at least 20 minutes worth of hot water if the shower head is putting out 2gpm.

Any suggestions on what the problem might be or where I should look next??? Could the shower stem valves (Moen) be causing the issue?

Install a lower flow shower head.
Good ones will still feel vigorous, they mix in air and give a good stiff shower.
Cheapos will just spray pathetically.

And yes, the Moen cartridge might be bad, and could be limiting hot or passing excess cold through the valve.

Change it just in case, it will save you a headache later when it gets stuck from old age.

Meanwhile, why the heck would you downsize the heater?

You should have looked elsewhere.

I have even had water heaters shipped from Amazon and far away online plumbing houses, free shipping, and better prices and options than Lowes or HD.