Personally, I use the FGs only for the gacha and storage expansions, with the first one having a higher priority. I expand only when I'm no longer comfortable with needing to sell something to make enough room to start a new mission. I use only the FGs found in the game - I'm not buying any.

The thing is, you want to have as many 5*+ knights as you can get.

In missions like the national ones or the whale ship ones you'll need a lot of stronger girls. The units below 5* may be enough at the start, but when the missions get harder, they'll need to be replaced by stronger ones.

Additionally, this is a game based on collecting different units. Since we have quite a number of different characters with their own voices and quirks, I'd think not using the gacha is kind of a waste.

Other than that, the Nutaku gacha got unofficially buffed, so it's easier to get good units off it compared to the original DMM one. It's actually one of the few (if not the only) advantages this version has over the other.