Connection Failure is a pairing problem, so it could be Bluetooth or LAN depending on what got disconnected. I have no idea if your laptop would disconnect Bluetooth when using an external dongle when you activate airplane mode (ie does it happen in Windows or does it turn off the hardware in the laptop). Connection Error Messages
My understanding is that BLE does not have hard connection limits. There are limits imposed by Bluetooth bandwidth and the processing power of the device. The processing power of the old laptop is not good. It is not performing well but I’ve seen worse in devices that are usable. I’m unsure if the weak laptop is a factor here, but I would be trying other devices to compare. I still suspect that your troubles are mostly related to the large number of paired devices.
I also don’t know what’s in that Bluetooth dongle you’re using, if trying something like a TP-Link UB500 would help, or if pairing via the Companion app would help since that would use the Bluetooth adapter in the device running Companion.
The other test you could do is to reduce the number of paired devices and see if things improve. Like do not pair the Rizer or cadence sensor and pair only one of the Zwift controls. Is the connection stable? If so that would help confirm that this is some kind of problem with handling the large number of paired devices.