My current game is a 769 Norse start, converting to merchant republic. It has been interesting.
My original idea was to have a merchant republic of Denmark, beginning at the count level. None of my several attempts at that level were sustainable—elective gavelkind takes a heavy toll. I eventually restarted as the old chief Harald Wartooth (duke level) and I've gotten to 1188 as of this writing.
When to convert is the tricky question as always. I built a large market city in all my direct holdings and in important vassal counties as well, but since I had expanded a lot there were some holdings that did not have this. I also tried to get any existing prestige buildings to level 2 or 4.
I probably could or should have waited longer, but I grew really impatient with elective gavelkind so I reformed the religion in 827 and formed the merchant republic in 900 (empire of Scandinavia formed in 868). By comparison, when I played an Irish count from the 769 start I didn't convert to feudalism until sometime after 1000 (or 1100?).
Some things I noticed along the way (that you may already know):
The intrigue option to upgrade holdings after you form the republic is just for holdings that are ready at that moment as far as I can tell. As other tribal holdings become eligible you need to do it again (although I also had a tribal holding automatically convert when the large market city was complete—perhaps because it was conquered and already had a stone hillfort?). This includes tribal lands you capture—if you don't upgrade the holdings by decision before you distribute titles you can end up with wrong holding issues.
I have found it to be necessary to hang on to captured counties that don't have a city until you have built one so that you can give the city to someone rather than handing off the castle and ending up with a vassal barony instead of a grand city.
Losing the succession election can be bad for your realm. The AI rulers are incompetent, and in my game they lost big chunks of land from the empire while wasting troops on pointless county conquests on the other side of the map. Try to keep control at all costs. Spend as much as you need to on the campaign fund to get someone fairly young elected, otherwise your princes will all be elderly and die quickly.
Holding multiple kingdom titles (serene republics) generates a lot of hostility and independence factions. I didn't understand why the AI destroyed kingdoms I had created when it got control of the republic, but I found out later and destroyed most of the ones I held after assassinating a bunch of faction members. Do this (destroy titles) when you're old because the opinion hit is substantial.
Without the ability to get the viceroyalty law (CA limited to minumum or low), the vassal limit becomes an issue in a large realm. I have a couple vassal kings just because of the vassal limit.
This is my first play-through as Norse. Expansion seems inevitable even if you don't intend it. In the early game local conquest and expansion seem almost an imperative, and later your vassals take care of it for you. Once I held most of Norge and Sweden I didn't need to do much conquering myself for my realm to expand. My duke-level vassals are fond of prepared invasions and holy wars, and I got Iceland, most of the British Isles, and some Baltic counties without needing to do anything on my own. The downside of this is that with low CA it becomes difficult to neutralize powerful vassals—I have several with multiple duchies, and one created a kingdom on his own that I can't take away because you can't revoke without cause.
In one instance when my character died I discovered my heir had launched a prepared invasion of Provence. I won the war but later granted independence to the duke because it was too difficult to defend and the various opinion modifiers weren't worth the trouble (foreigner, religion, wrong goverment type). Another vassal family pushed its way up the Elbe into Pannonia and down to the Gulf of Venice (and I actively took territory in Poland and Bohemia), so now my realm reaches from the Baltic to the the Mediterranean. I recently captured Venice and am looking to steal some trading posts. . . .
Fabricating claims is very expensive.
The Fylkirate is subtly powerful. It gives you prestige as a duchy but doesn't add to your duchy count for purposes of vassal opinion. It's really good for your religious vassal opinion. You can do serious damage to Catholic moral authority via successful holy wars, crusades ("great holy war"), and county conquests, and looting Catholic temples. This leads to widespread heresy—most of Ireland and much of England were Lollard when they were conquered. Throw in an anti-pope or two and Catholic crusades and holy wars aren't that big of a threat (we just beat Francia and the pope in their holy war for Germany, held by our Saxon ally).
Read war conditions and outcomes carefully. I launched and won an expensive and hard-fought great holy war for Poland, and it all went to some nobody instead of to me; he became the 14-county duke of Greater Poland. The king of Poland now has only one county and is being attacked by an adventurer. I'm going to take the county from him and I'm wondering if that will let the upstart duke usurp or create the kingdom title.
Notable dates:
813: created kingdom of Danmark
827: reformed religion
868: created empire of Scandinavia
887: completed a prepared invasion of Provence begun by heir
900: converted to merchant republic
933: lost control of the Principality of Scandinavia and Serene Republic of Danmark
935: recreated Serene Republic
942: regained control of Principality
959: lost control of the Principality and Serene Republic
975: recreated Serene Republic
982: regained control of Principality
1172: usurped Venizia
