“e-3.2-a15-20251026-CUSTOM-channel.zip” boots fine (despite Signature verification failed E:error 21 - probably up to the mismatch of the recovery I used) - initial interaction works, thanks ronnz98!
“e-3.2-a15-20251026-UNOFFICIAL-channel.zip” being 300MB bigger than the custom 1GB file is probably already too large just for the sideload transfer? ends up in a 28 (ErrorCode::kDownloadOperationExecutionError) .. Installation aborted.