Thousand thanks for all the advice!
In the end, I've decided to start from scratch and do an evil playthrough of PoE (which I've never finished) as a Bleak Walker until Deadfire difficulty is fully patched up, then try to transfer it as a Bleak Walker/Troubadour. I'll be using companions to fill in the gaps in the Death Knight/Blackguard skill set (mainly frost magic, plague).
Thanks to you, I already have ideas for the next character though! Have fun