pesarek84, thanks to your contribution i believe i was able to resolve my issue - only after doing due diligence and trying what everyone else said, short of completely erasing and resetting back to factory settings.
this seemed to be a multi-prong issue for me -
1.
my iCloud was full, and i had put off upgrading my storage account. when i asked a friend about the blackscreen/spinning wheel issue, she said she heard it has to do with iCloud. so i went in and upgraded.
2a.
followed pesarek84's instructions, and attempted to backup before deleting, but it kept blackscreen/spinning wheeling so i wasn't able to clear it all out.
2b.
then, i wasn't sure if i had iOS 11 installed, thought maybe i was on 10. found that i did have 11.1, but also found the 11.2 upgrade. started the installation process for it, however i needed to backup my phone, and had trouble doing so since the blackscreen/spinning wheel thing kept happening, restarting the process.
except that this time, when it would 'shut off', i'd go back and try to install the iOS update, and luckily it seemed to pick up where it left off. once the download was complete, it rebooted itself and now seems to work fine, with no blackscreen/spinning wheel action.
--
in the past, the sleep/volume down button seemed to have some cache clearing action, as it would make apps run faster but it didn't help at all with this issue.
hope that dl'ing iOS 11.2 works for those still affected!