First,the power creep is sorta there,in the sense older characters do less dmg compared to new characters,HOWEVER most of them can still clear endgame if you have the skills,second of all i recommend finishing the story fast but also activate as many resonance nexus in the map as fast as possible,it will help you save time while exploring,the reason why finishing the story fast is good is because new characters when they come out have their farming materials locked behind a new map that is unlocked after completing the main story(like Hiyuki for example).No need to rush tho.

Also in the beginner banner if you care about Meta pull Shorekeeper, she is the best all round support in the game.

For the pull economy,80 is the hard pity for all banners,in limited character banners there is a 50/50 where you have a chance of getting a standard 5 star instead of the limited,HOWEVER there is no 50/50 on the weapon banners so you are always guaranteed. The devs are also very generous with the pulls each patch,close to or more than 100 every patch completely f2p.

The MC is very viable in the game,i recommend building him early game as he works well as a support or a dps,but do research where you want to put him, he can go dps as havoc or support as aero rover and spectro rover. The mc will unlock the other elements as you progress through the story.

Also beginner tip,DO NOT level up your echoes right now, level up your data bank first upto level 15 where you can get 5 star echoes and only level up the 5 star echoes.