Same problem here - brand new iMac Pro - £5000 worth of computer with *terrible* Bluetooth performance. I only use Bluetooth for the mouse (Microsoft) and a pair of Sony WH-1000XM2 headphones; ever now and then they both stop responding at exactly the same time. The mouse pointer freezes and the sound goes to the iMac's speakers. 30 seconds or so later, the mouse always comes back, the headphones might need to be reconnected. A little later, it happens again. In between, the mouse is sometimes choppy or feels 'sticky' as it lags behind on the screen. When I try to use a pair of Sony WF-1000XM3 earbuds with the Mac, the sound is choppy and thin and they regularly disconnect from the Mac.
I'm not using a wireless connection so it can't be interference between WiFi and Bluetooth (which would be a massive failure on Apple's part); the batteries in the mouse have been replaced and the headphones are fully charged. I've done all 8 of the steps suggested above - including resetting everything, re-pairing, reinstalling MacOS, updating drivers on the devices and the problem won't go away.
All of these devices work faultlessly under Windows and iOS, so either Apple is shipping machines with faulty Bluetooth modules, or there is a bug in Catalina. I've reported the issue to Apple but their advice is just the same as has been shared here. Hopefully enough people are reporting similar problems for them to be working on a fix.