Factory reset the controller and drone, pair them, upgrade to Opensolo, Then pair them again.
sidepilot firmware should take you up to opensolo 4.0.0 on the controller and copter, do the copter first, then the controller.
Open sidepilot, top left drop down bar, help and settings, 3dr solo setup, 3dr solo firmware upgrade, download opensolo 4.0.0 clean install controller and drone, copter first, let it finish which takes a while and ends in a permanent disco of rainbow lights. the controller is much faster, press twice once downloaded to affirm installation.
sounds like you already did this step?
Once you have completed this (there is a whole procedure with disco lights) the copter will be updated, the controlled will update twice as will copter.
Pair your copter to controller and open with opensolo,
I have issues with compass calibration once you get the controller and drone hooked back up together.
Factory reset on drone, press and hold the pair and battery power button with the drone off till you see one of the orange lights rapidly blink, if you are looking at the drone from the belly and the power button is in the bottom position, the blinky light would be the left one.
Factory reset on controller: press and hold power and fly buttons for duration of time till controller starts reset protocol
There is an option in sidepilot to reset the factory firmware to the opensolo 4 firmware, As the upgrade to opensolo takes time there is a good reason to do this.
I hope your 3DR works better than mine at this point, Once I opensolo'd both the drone and controller my sidepilot would no longer connect the camera but I have a feeling this has to do with the magnetometer.
I find it hard to throw these drones away when the hardware becomes unsupported, a $15 dollar sensor making a Previously $1k unsupported drone chassis completely unusable is such a 2020 planned obsolescence thing.