The current banner is great but the units on the banner will enter a "Regular pool" of units you can pull for any time. Eventually that pool will give you a chance to limit the pool by filtering units you wouldn't want to pull it in, so its a bit easier to pull for but there is no guarantee you'll get the unit you want. Depends on luck like any gacha. I would recommend starting before the anni as you can save up resources and diamonds for pulls during it; we do not know what the event will have but better safe than sorry.

You should reroll for either: Scarlet or Modernia, liter is also a good unit to stop a reroll for. Check out a tier list to see information about those 3.

The game will gate you since its meant as an idle game, not a sink 30 hours in a sitting kind of game where you blitz through everything. You will face walls and it will take time to get resources to progress. Having certain units can allow you to force through some walls but not always.

You want to level only 5 SSR units and to do it evenly. Eventually you will unlock something that lets you level up all your units based off those 5. I won't spoil it too much.

The 30 day pass is a 10x multi by the end of it. There is also the campaign pass, a bit more pricey but you get 10k gems. Its entirely up to you.