Thanks for all your details.
The Bluetooth situation is pretty weird, since normally BT pairings should not be lost during the update, and to that, we add a connected watch that connects (apparently) without a pairing.
What’s more, you can’t re-pair the sensor, I presume for reasons of medical integrity. Given the current state of the BT configuration, in normal circumstances I would have advised deletion of all BT and Wi-Fi network information (a “network reset”) so as to start afresh with new pairings.
I think it would be a good idea to take the advice of the sensor manufacturer, and probably that of the pump manufacturer as well.
I would tend to agree with @anon66320559 in their post in the other topic, about instability of apps running on some FPs after the A. 15 update. Have you encountered problems of apps not running properly in the background?
Make sure the apps have unrestricted background permission:
Settings > Apps > [your app] > App battery usage > Allow background usage > Unrestricted