FYI ... Got it working.
Spun up a fresh Win19K VM with a SQL17K.
Allied all the usual changes including the guidance mention throughout this thread. Still had the same Auth error issue .... but this time I changed the actual server properties (not just at the DB level) which by default ony allows Windows Auth instead of mixed mode.
Had a minor new issue with the /forum/database folder permissions. I did set the IIS user to read/write/mod ... but kept failing to update the config file during setup. Had to temporarily add the Everyone group with FULL permissions to get it to work before removing the group again. This could be because the default file perms for Windows Server 2016+ is different to older versions of the OS and certainly different to client OS (Win 7 and above) which has the Authenticated Users groups in there etc.
I've captured a set of bullets to get it working on an isolation environment like mine if you want to poilish up the out of dat KB.
Thanks for all your heklp Bruce!