max you can get without raiding is 28 I believe - thats full purps in every slot, and an exotic armor piece you buy from worm head, and every armor piece fully upgraded (need lots of ascendant mats to do that). I don't raid and I'm at 27 and would be at 28 if I went out and farmed some spirit bloom (yeah i know). At that point all you are really doing is waiting for xur to drop different items and spend your coins and light motes on them to maybe tweak your stats to be more ideal for your build. 100% doable with just cave farming, PVP, and public event farming.
Once you hit rank 3 in vanguard and crucible you can wear armor tabards and farm up to rank 3 in the other 3 different factions (dead orbit, etc) although again the only real difference is different stat balance. I think you get a "free" purple when you hit rank 3 of a faction although I dunno?
The story is just a hot mess, no denying that and thats why its reviews are all "meh". Its even more shocking that the grimoire didn't almost even make it, a designer posted somewhere that they raced to do it in the last few weeks before launch and made it sound like some noble heroic act. Jesus Christ. I'm pretty sure the only reason the grimoire isn't in game is because of the ps3/360 port, for a xbone/ps4 it woulda been easy to put in a webpage load but not so easy on older gen.