Terrible bluetooth connectivity after upgrading hard drive to SSD

Did you take the display off? If you have you may have damaged one of the antenna units which are placed around the displays edge.

Frankly, Your system should still be under warranty. I would just let Apple fix it.

From my understanding the 2019 model has an intergraded WiFi circuit like the newer MacBook Pro’s so you can’t replace the AirPort board like you could in the older 21.5 & 27” models.

Easiest fix is usb-bluetooth adapter (get a usb-hub too if you don’t have a spare usb-port), I’ve used usb-bluetooth adapter when bluetooth-electronics have died. But since you have a weak signal, that really does sound like an antenna issue, not electronics issue. If that iMac is going to be opened again, verify blue-tooth antenna cable plugs look ok with center-pin not bent, and are antenna cable tiny right-angle plugs are fully seated correctly on tiny coax jacks on logic board. Make sure the little black coax antenna cables look ok: not crimped/knicked. After re-seating antenna cable, test blue-tooth while you still have it apart. Best to do bt tests with no other external devices connected except a usb-keyboard and usb-mouse… temporarily turn off your bluetooth keyboard and/or bt mouse while testing.. You might install homebrew then do brew search blue to find utilties to help do bluetooth tests, like bluetoothconnector  blueutil and others. I’ll post back here if I find an easy way to confirm bluetooth signal quality (strength, s/n ratio, etc.) If you have a spare bluetooth antenna with the same type of plug from an old dead laptop/desktop, you could temporarily connect that in place of the iMac internal antenna. If bluetooth works with temporary antenna, then you may need to repair/replace the bluetooth antenna inside your iMac. Or just skip all this troubleshooting headaches, get a usb-bluetooth adapter, and turn off the internal bluetooth forcing it to use only the usb-bluetooth adapter for all bluetooth connections.