I've been a fully verified customer for several years though so would not expect them to go so far as to freeze my funds without any notice
You certainly wouldn't be the first person it had happened to, and Bittrex certainly wouldn't be the first exchange to do it. Plenty of verified users have their accounts frozen while facing demands for "enhanced" verification, proof of source of funds, employment information, etc.
they even asked to write a word bittrex on a piece of paper with the current date of taking a selfie with the ID and a video and that's bullsh*t to me, so stop after they asked.
That's actually less scummy than asking for your employment history and all the other ridiculous details that OP was asked for. I would completely avoid performing KYC at al all, but by asking you for a selfie holding the word Bittrex and the date, they are simply trying to verify that you haven't stolen or bought someone else's documents. The information OP was asked for, on the other hand, is way over the top. I've given away less information to take out a mortgage.