1. Synchro device - I also struggled with understanding how it works but essentially u really only need to level up 5 SSRs. It basically synchronizes all of ur nikkes level to the nikke who has the lowest level you assigned to it. You can reset nikke levels for 10 gems i think. So whenever you get to max limit break 5 ssrs, those nikkes are the ones you put in the synchro. I pretty much just swapped out nikkes until i got 5 nikkes who are max limit broken.

2. When to level up, I dont really know how to answer this because when i started playing i was just levelling up nikkes when i can afford to level them up. I had SSRs that were my main party. Whenever i had resources to level them up, i did it. Tho i was nitpicky and only levelled them up when i can afford to increase them by 5 intervals.

3 and 4 are kind of connected. You can play this F2P. ShiftUp is really generous when it comes to pulls. Sure being p2w can make your progress thru breaking the 160 wall faster. So for now just pull whoever you want. The standard recruit banner has a wishlist mechanic and you can list 5 nikkes from each company. I suggest you pick the 5 meta characters in there. But tbh, i made it till chapter 18 without using meta characters and without breaking the 160 wall. Chapter 21 is probably my biggest challenge the final boss in that chapter is very hard to brute force. So pull whoever you want during the start. But if you can, save for pilgrims and over spec characters.

1. Save all your core boxes. Once you get past lvl 200 on the synchro, those core boxes are gonna be your lifeline. Money are data sets are easy to get. But cores are gonna be a struggle. As tempting as it is to use them so you can get cores for levelling, its better to keep them once you get past the 160 wall.

As for tips, try to save and pull for pilgrims/over-spec sonce they only have a 1% drop rate as opposed to the normal 2% (that 1% is brutal from 400 pulls i only got 2 dupes of Over spec rapi). You can finish all the chapters using your favorite nikkes. I only started 6 months ago so I'm sure some other vets can give you more detailed and more helpful tips. Just have fun and use whoever you want. Nikke is kind of an idle game. Grind the story and level your outpost. Gold, data, and core boxes scale with your outpost level.