I havent found anything online at all about this, so i made a strat myself. This is obviously with the gamerule Dynamic Religion.

Ill write this down step by step, its a bit RNG based but in the end you should either become norse, or if they all die in the plague, you're atleast going to be sapmi

• Ally the Teutonic knights day one (if this is an mp game) or no cb lithuania
  
  
• Remove all clerical privlieges that increase pop conversion (Enforced Religious Unity)
  
  
• Add protected Tribal faith privilege, idk if this does anything rly but i do it never the less.
  
  
• To keep a mark on how you're doing and what you can do further, check the monthly conversions in any state with Norse, we should aim for 0 to keep them around longer. But at the start of the game, 1 is good enough

• If this is MP, just help teutons siege down Lithuania, do it as quick as possible and make them transfer it all to you. Move on to next category
  
  
• In SP, you will have to wait for lithuania to unsiege the provinces that the Teutons start with, this might take some time, its entirely RNG based. After that, just move all your troops in and siege until 100% warscore.
  
  
• The war should at best only last a year or two, depending on the sieges it might take way longer.

• Try to aim for as many provinces as possible with Romuvan faith, higher pop is also good, i.e:
  
  
  Try to take modern day Lithuanian borders basically

• Right after the peace, release basically all your southern provinces as vassals, something like this:
  
  
  
• Aim for 40% of your pops to become Romovan, and then convert to the religion in the religion tab.
  
  
• After this its basically RNG, we need to wait 10 years and try our best to lower the conversion rate of the norse people
  
  
• Try your best to not kill all the norse people during the black death, since it has a habit of erasing them from the map. This is why the Lithuanian war should be as quick as possible, since the black death spawns around 1346. I managed this with just 70 or so norse left in the year 1349-1350 since the black death didnt arrive yet. You can try to limit trade during that time and so forth.
  
  
• You can get your land back pretty easily, just go to your vassals and press the 'return land' button, you get everything except the capital.
  
  
• Since norse and romuva is in the same "religious category" you can swap to them without this requirement of 40%, it works the same as swaping to i.e orthodox as a catholic nation.
  
  
• Extra tips for this is to keep one of your cabinet possitions on Stabilizing the country since you're gonna be very low on stability for a long time. Try to raise your humanitarian value as high as possible by changing laws during parlamentary debates.

This entire strat takes about 11-16 years, mostly depending on how fast you win the war with lithuania. And if all your norse pops die, you can always chose to become Sapmi Sweden :)

Credit for this goes to me (Male335) and ActualEmil, we did so much testing and it took us 3 days to get to this strat. If anyone has any ideas on how to improve this i'd very much appreciate it :)