I just updated to Iode 5.13. I had some issues, but I could solve it.
- I backuped my apps with android_backup_project and sideloaded Iode 5.13 with wipe cache / Dalvik cache as usually
- When I rebooted the phone, my EFS was corrupted. SIM was not detected and Settings → About Phone → IMEI did not show my IMEI anymore
- I first tried to restore the corrupted EFS with the adb method from here, however this did not work even with root debugging activated and adb in root mode (the command was running successful, but the IMEI still did not show up after reboot)
- I then tried fixing EFS with the TWRP method as described here and that has worked for me before.
- EFS was fixed then, the SIM card was detected again. However, IMS features (VoLTE / VoWIFI) did not work even though in Settings → Xperia Parts the right modem seemed to be used for my provider
- I could fix this by Settings → Xperia Parts → deactivate IMS Features → restart → Settings → Xperia Parts → activate IMS Features → restart
I am now in Iode 5.13. with latest security patch from May 2025 and VoLTE / VoWIFI is working again. Thank you @petefoth for the build. 
@MaxO
I also understood that Android 15 / IodeOS 6 is very unlikely to be ported for our device as the available ports are very buggy. However, I am fine with the device now with Android 14, there are regular security updates thanks to the builds from @petefoth (you need to install them manually with adb sideload), unless of the corrupted EFS problem occuring occasionally (where I know the workaround to fix it again now) the ROM is super stable, and it is expected that Android 14 will be supported until March 2027 with security patches, so I am happy at the moment.