If they don't know SS, and they're also a commercial gym, then my experience of thousands of cases is so far 100% that they also don't know what they're doing when it comes to form. So you might want to get a second opinion here too, by getting verified and then posting in the coaches Q and A for a form check.
You can probably switch the DL from 3x weekly to alternating sessions. On the days you don't DL, do barbell rows and assisted chins and/or lat pulldowns, so you start getting some more upper body work in. You can buy your own micro plates and bring them to the gym with you, this is what my clients do who train at commercial gyms: Dan Miller Microplates or Amazon Microplates. Use them to make 1-2 lb jumps on press or 2-3lb jumps on bench. Once you stall again after you get the plates, switch from 3 sets of 5 to 5 sets of 3 reps and keep adding 1-2 lbs per workout as long as you can.I am still making 5 lb increases on my squat and will be switching from 10 to 5 lb jumps on my DL as I am noticing the recovery slowing down.
I did do 5 lb jumps on my overhead press and have gotten from 25 lbs up to 45 for 3 sets of 5. Unfortunately I haven’t improved. I think the same thing will be happening for my bench soon (up to 55 lbs there).
What can I do to keep making small gains on my OH and bench presses? My gym doesn’t have weights smaller than 2.5 lbs. my guess is that I should deload just a bit and increase volume. Should I also start doing assisted chin ups?