You don't have to spend a penny of real money to be competitive in the story and general questing stuff. I've been playing it since October without any investment. You get premium currency through basic quest completion, and for a million other things, so you'll laways have a steady income as long as you play every day. In addition, there's the ability to acquire otherwise gacha-exclusive characters via playing in-game events (there's one on right now) where all you need to do is acquire a certain amount of points through completing in-game challenges and just buying the character. Characters rotate in and out the gacha shop as well, so there's the possibility to just buy a character later on. You will get a full roster of starting characters just by unlocking the main quest.

As for the rest, it's a lot of making your own fun exploring, opening chests, and solving puzzles, with a lot of grinding if you want to go pro and compete in the Spiral Abyss or the high level domains. This is almost entirely optional.