Are you using ethernet backhaul or wireless backhaul ?
That load should not be an issue for the single wifi router. The client determines where it wants to connect.
The first thing to do is make a wireless strength map where each of the clients are located. Turn off the second wifi router.
Use something like Apple's Airport Utility and change in settings to allow wifi scan. Do a 20 sec scan at each wifi client. Write down the signal strength at each location, channel number, for each SSID visible. Record for both 2.4GHz and 5 GHz and 6 GHz ( if you have) . Ignore any below -75 dB
Wire as many of the clients as you can with ethernet cables.
The issue is most likely on the 2.4 GHz band since it penetrates walls better. Reduce the TX power to the lowest possible setting on the main wireless router.
Go back and do the survey. See who drops below -65 to -67 dB
If all the clients see -67 dB or higher (smaller number) in the original survey , then you probably don't need the second wireless router at all. Set your 2.4 GHz channel width to 20 MHz or 20/40 if no interference from neighbors. Set the 5 GHz channel to 80 MHz or lower. Call it a day.