I ran into this exact issue early last year and figured out the registry setting to re-enable thumbnail view. I am sorry to say I cannot remember exactly what I did but I figured it in OSOT using the "My Templates" tab where is basically shows you exactly what registry changes are made for each setting. I left notes I believe in the OSOT comments section but cannot find them. However maybe this will get you on the right track: https://blogleodegroot.wordpress.com/2020/03/07/explorer-thumbnail-view-disabled-after-running-osot/