Hello,
To keep you updated on my next steps: Following the advice from customer service, I’ve now performed a factory reset, which was successful: In addition to my settings (of course), all the junk data is gone, and the device is fully functional again:
Storage: 17%
WiFi: ok
Mobile Data: ok
Phone / SMS: ok
Bluetooth: ok
Battery: around three days
Fortunately, all this effort also led me to Android 16, since I had already installed the OTA update IMG-e-4.0-a16-… linked by @piero locally. I didn’t need to use the web installer, so I can’t comment on it (but thanks anyway for the detailed instructions!).
All in all: It took quite a bit of effort, but it was a success, and I learned a lot in the process. So thanks to everyone for your tips. 
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