Nope...full UEFI, no legacy, windows on a GPT partition, and Safe Boot enabled. The whole ball-o-wax. Still nothing that says 'fast start' in BIOS. But the BIOS doesn't even have a Compatibility Mode, or at least nothing labeled as such. Instead it has a 'windows enhanced' mode. Like 386 enhanced mode I thought? nope. i'm pretty sure DISABLING that is just compatibility mode. So in a similar fashion, Fast Boot could be hidden in some other setting with a name I'm not catching the drift of.
And I can agree that Windows' prefetch, as "fast startup" used to be called, is dodgy pile of...whatever. It's always breaking and unbreaking on my system; it seems with every update pushed by Microsoft. But when it works...it is sweet with very much faster startups, although shut downs can be extremely tiresome if I don't quit all open apps first. But I didn't recall OP limiting his quest to BIOS tweaks only...he said something about "anything I can do". That is one thing.
And if you want to disable it, just unchecking it doesn't quite do it. You have to disable the service too. Good luck finding it.. they change the name to keep it a guess.