Struggling with a fresh install here.
Bootloader unlock was yes, got it unlocked okay. TWRP installed and running.
Firmware was already 23.5.A.1.291 as mentioned here
https://wiki.lineageos.org/devices/z3c/install
Flashed lineage-16.0-20200714-nightly-z3c-signed.zip and also tried the oldest one on lineage site lineage-16.0-20200325-nightly-z3c-signed.zip
At first I tried with gapps/su but additional attempts were all tested without.
There is no error during flashing, when rebooting lineageos boot logo runs for a few minutes then it reboots back to recovery.
Done the cache/system wipe as mentioned. I've never had a flash that failed at the boot logo and return to recovery/twrp itself before.