Just to clear up all doubts about the no title creation bug, I ran a game in vanilla. A few decades of observation confirmed that it is indeed a vanilla issue. The hands-off game also made me realise why kingdoms are created quite often in vanilla and thus why the issue went relatively unnoticed in the main forum: because of all the lame CBs the pagans get! For example, there's the "Become king of" ambition that allows you to essentially snap up a whole de jure kingdom in years with limitless subjugation casus bellis. That CB will take all ducal titles as well - for example, I witnessed Holmgardr conquer the majority of "Rus" with this casus belli. Most of its enemies were count-level or only had a tribal title, so Rurik didn't gain a second ducal title for a long time, but eventually it fought a de jure duchy of "Chud", subjugated it and gained the ducal title, immediately forming "Gardariki". Similar story with Denmark and Sweden. On the other hand, Sudreyjar never managed to get a second duchy title this way, so when I stopped observing, Sudreyjar had 500ish gold, nearly all of Scotland, half of Ireland and in a good vanilla fashion, a patchwork of provinces all over the coast of England - but it had no chance of forming any kingdom.
Since the turbo-subjugation CB is taken out in CK2+, this cheap and easy way of forming kingdoms is gone. As it is the ONLY reason kingdoms form in vanilla as well, in CK2+ it is understandable why no pagan kingdom will ever form starting from a single duchy. It'll be weeks before we have a reasonably playable version of TOG CK2+.