Is co-op worth bothering with?
I like it. Tho I had to be carried by my siblings until I got enough good artifacts. I'm at world 8 now where I am comfortable enough to carry others tho I still ask for help every now and then. Usually if I go to a world, I ask the host if they need anything so yeah gotta use the chat feature. Tho you do get those who go to your world just to raid your resources. If you're fine with it then okay but I you don't want them to, say so and maybe even kick them out.
In terms of gacha, if you plan to play F2P like I'll be doing, is the way to get currency (fates or whatever) just through spending primogems at the Paimon store (or whatever it's called)? I assume all the basic "gacha strats" apply. AKA, save your shit til there's a banner with something you want and then do ten-pulls. Are the weapon banners worth doing though?
You can get fates one time as level up rewards or from statues and the special trees in Dragonspine and Inazuma. Otherwise, you are right. You gotta convert the primogems which you can get daily. Lastly, imo, weapon banner not worth unless you whale. There are good f2p weapons and for the most part your damage will come from artifacts and talents.