A small update - the d3d9.dll that I had came from some visuals mod that didn't get wiped when uninstalling the game via steam, I've unsubscribed from all the workshop content there was, and did a manual folder wipe, deleting everything. At first windows refused to let me delete the .dll file, stating it was being used by mb_warband.exe, despite no proccess being active, however a quick reboot allowed to delete the file. Did a fresh install on Steam, and the game seems to work without a hitch!
Huge thanks for your time!