My experience is that Zigbee’s range is fine, but it is highly susceptible to interference/electrical wires/water pipes that might be in walls. I put my Conoff dongle on a 12 ft USB extension (my HA Raspberry Pi is right next to my router, modem, and home server). Then, as I was having issues with devices staying connected, I added routers. I tried to place them so they had clear line of sight with each other (or the dongle), or only through one wall. After that, I’ve had no problems. It ended up being about one per room. Ikea’s smart plugs are cheap and I use them only as routers.

Do you have any zigbee routers (mains powered devices) scattered around your house? I had the same problem, then I strategically placed a bunch of smart plugs throughout my house. Haven’t had any problems since.

While I don’t have the number of devices you do, I can add two observations:

Kwikset Zigbee Door Locks have been rock solid since the day I installed them. No issues dropping off the network. The only downside is occasional delay in reporting state (locked/unlocked) to HA. Getting it to respond to a command to lock/unlock is near instantaneous.

Ikea Zigbee devices have also been rock solid. The only issue is that none of them seem to report battery level reliably. Other than that, there are zero issues.