There are various workarounds for elective Gavelkind, som of which are more exploity than others, eg:
- Only have one son (separate from wife and concubines after he's born)
- Only have one living son (contrive to get the others killed, before they have kids)
- Only have one eligible son (use seduction focus to create a horde of bastards, pick the one you like)
If you can, the "easiest" way out of elective gavelkind is to reform the religion in your first ruler's lifetime. Look at the
map of holy sites, and pick a ruler who has easy access to at least 3 of them. (This tactic is easiest for Germanic rulers and hardest for Tengri/Suomenskuo.) Fabricating claims with your chancellor may be useful.
If your realm is going to split from gavelkind, pick the whichever of your sons is the best general as your successor. (That's the "elective" part of the succession law.) All of your sons will get strong claims on your other
independent sons' top-tier titles. And a strong general is best placed to reconquer them.
I usually prefer to save my once-per-lifetime subjugation CB for a few decades after game start, after the AI have blobbed up via their own subjugation CBs. You can use your "free" subjugations from "become king" before your lifetime subjugation, if you're careful. (Find a nearby realm that's 100% outside your kingdom; check you can in theory subjugate them immediately after every subjugation declaration inside your kingdom; if not, savescum.)
"Extort tribute" wars are a very useful opener for pagans - these tributaries are forced to join your wars; and you don't have to manage them like vassals; and, after 5-10 years, you can break the tribute and instantly declare war on them. Basically, they are a very cheap force-multiplier: having 2-3 tributary duchies means you can stomp any other nearby duchy with ease.
Slavic is a defensive pagan religion, which means you get
massive combat bonuses in counties that follow your religion. (It's the county religion that matters, not the religion of the count/duke/etc.) If you're fighting a non-Slavic realm, make sure the battles take place in Slavic counties. Be aware that Suomenskuo and Romuva get the same bonus, so do not ever fight battles in their counties.
Join the warrior lodge and work your way up to the top rank ("hero") asap. (Don't spend society currency on anything except level-ups, and do the missions asap.) That top rank gains society currency very fast (so you can very quickly do all the fun things you skipped at ranks 2 & 3), and it also has the ability to instantly refill the levies of any settlement (very useful for recovering from a war).
There's probably more I could say, but I think that's more than enough for now!