My Controller Drops Out Whenever My Phone Is On The Desk Next To My PC

That is excellent diagnostic work, and the mechanism is straightforward once you have found the pattern.

Your phone is a 2.4GHz transmitter. Its Wi-Fi radio, its own Bluetooth connections to a watch or earbuds, and any hotspot or file transfer all operate in the same band your controller's Bluetooth link uses. At a distance of a few centimetres from your PC's Bluetooth adapter, the phone's signal arrives far stronger than your controller's does from across the desk, and it simply drowns it out. Bluetooth has frequency hopping to cope with interference, but a strong transmitter sitting almost on top of the receiver overwhelms that.

The thirty-second delay you noticed fits too: it is usually the phone doing something periodic, syncing, scanning for networks, or waking its radio, rather than transmitting constantly.

The better fix than policing where your phone lives is to move the receiver. If your Bluetooth adapter is a USB dongle, put it on a short USB extension cable and raise it ten or twenty centimetres off the desk, away from the back of the case and away from the surface where your phone, keyboard dongle and cables all live. That single change resolves most of these, because it gets the receiver out of the noisiest layer of the desk. If your Bluetooth is built into the motherboard, its antenna is at the rear of the case among all the cabling, which is one of the worst possible positions, and a cheap USB dongle on an extension often outperforms it noticeably.

Also worth checking: the adapter or dongle should not be in a USB 3 port, since those leak noise across this band, and it should not sit next to a 2.4GHz mouse or keyboard dongle.

If you want the problem gone entirely rather than mitigated, the cable removes the radio from the equation altogether and gives you lower latency as a bonus.