Sorry, my image just shows the default name when first create a Custom Scan it will be called a Quick Scan, perhaps I was wrong in (not reading the full topic) thinking you had created a custom scan.
Why this isn't available if you open the Protection > Virus Scans - Full Virus Scan settings tab is beyond me. This new UI is a bloody minefield.
I just wonder if you had this Full Virus Scam scheduled on a prior version of avast and the avastUI and that has been inherited on the existing installation and no way of stopping it.
I even checked the avastUI > Settings > General > Troubleshooting - Open old settings - General > Special Scans and the only thing there is the Explorer Scan.