So since you have the recovery already on your phone, try a flash without the bootstrap option again.
I already did that, tried it again now (also using a newer usb cable, as well as different usb port on the back of my pc).
The phone should reboot to recovery again and the UBports installer should start pushing the files onto the device.
It never starts doing that as long as I try to install the stable release again, which it has already downloaded previously. That only gets the installer stuck at "Rebooting".
Now I downloaded your dev release, and the phone seemed to try booting a couple times, unsuccessfully, while the installer was downloading UT, and got stuck on the Sony logo again. Ubports installer displayed the "Connection lost" alert.
adb shell commands are not giving any output, sadly:
[ben@catull-mnj ~]$ adb shell ls -lah /cache adb: no devices/emulators found
I'm wondering if there's something wrong with the phone's usb port maybe...
Is it a possibility to save a UT image to an sdcard, and have recovery install it from there?