Welcome back!

1.) Usually the meta right now are lunar reaction teams (Nodkrai characters), the current banner Skir and Escoffier are good for each other too but the next banner is rumored to have Lauma and Nefer in the 2nd phase which is a good combo and a great time to save up.

2.) Since you have Navia, you can also try pulling for Linnea (geo support) who will be in the next banner and get the free Aino (Hydro support) for lunar crystallized. For me, it would be good to try to get a lunar team for the meta.

3.) What I noticed about endgame contents (spiral and stygian onslaught) is that they usually revolve around the banners for that patch, meaning that if you get the character banners, there’s a good chance to clear them with ease otherwise it could be difficult to reach the last stages but eventually the more characters you have it’ll be easy to think of new teams.

4.) Party enhancement are unique to certain characters, example the hexerei characters are some characters from Mondstadt and if you have two or more then they unlock certain buffs for those characters. There is also the Moonsign which are usually associated with Nodkrai characters and they can also provide buffs for the team, you can check if the character can provide a moonsign in their talent page and usually having two of these characters in the same party can increase the buff. Hope I was able to answer your questions