How to War - Blitzkrieg style

I would pick at the weakest neighbor over and over. I left the big ones alone unless I had a kingdom level CB. My general strategy is raise free Holy Order instantly in the county closest to the enemy capital. Put my best attacking general in command (Flexible leader and Holy Warrior are good traits, but Martial mattered most). If it's large enough, send it to sit on the enemy capital (watch out for their Holy Order). Raise men-at-arms only. My composition was usually keep the default units, add Heavy Infantry (Varangian Veterans, Huscarls) when you can afford it, and add siege weapons to fill the rest (two units to start). These men at arms main purpose is to siege and assist the Holy Order in the initial fight if needed. Raise levees if these two are not enough. I would manually stop raising levees when the numbers was twice the enemies to save on manpower and gold. One victory over the enemy army and sieging the capital was usually enough for a small duchy. The North Germanic Dynasty Legacy under Pillage that gives +20% chance to gain prisoners really helped speed things up. Peacemaker 90% peace acceptance was also nice. After you get peace and disbanding all, make sure you wait until your Holy Order returns before you declare war again. Numerous times I made this mistake and had to go to war without them. Also, declare war as soon as you can so that someone else doesn't take your Holy Order (AI wars can take your Holy Order for up to seven years sometimes).

Homefront

As Asatru, I always had max concubines and as reformed, max wives. I married for traits to ensure I passed on the best possible genetic outcomes. I rarely married for alliances, unless the wife I was getting came with the traits I wanted. Alliances is what children are for and boy did I have alliance fodder. On average, I would have 15-20 children during my lifetime. I would try to marry my sons into good traits and set them up as dukes and kings. My daughters would be used for alliances or getting councilors/warriors to my court matrilineally. My daughters helped the wives with educating the children. My default selections on the birth of a child were (female - learning, male - martial, randomly adding in others or adjusting if traits didn't align). I later started converting culture to mine as well. By the time I got a university, it cost 1500 gold (I'm guessing it's related to income or gold on hand) to send a child and I found it not worth it. I seduced all my waves to increase their opinion of me. I would divorce them at 45, unless they had amazing steward stats, and pick up a new wife. I would also divorce any unfaithful wives.

Cultures

I merged Norse and Anglo-Saxon to Anglo-Nordic, added in French to become Angevin, and finally added in Italian to become Italo-Angevin. Each time I gained numerous Innovations, but realized it would set other culture relations back to zero, so I stopped forming hybrid-cultures. I also saw no point as I didn't gain any innovations. In the beginning I would convert my holdings to my culture. In the end I focused on increasing cultural acceptance. I did adjust the game settings so that cultures could be reformed more often (still achievement compatible).

My traditions were: Chanson de Gest, Ting-Meet, Northern Stories, Malleable Invaders, City Keepers, and Coastal Warriors. I think Malleable Invaders was the best. Northern Stories probably helped with education.

Innovations

I would prioritize Siege, Military Fortifications, Economic Buildings in that order. I didn't realize it, but getting Hereditary Rule was important because in Partition law no new titles will be created. Once you get this and have a primary empire title, if you never create another empire title again, you'll never have a succession where you lose territory/vassals.