Well, first point - it's kind of annoying to do. Second point - it will give you some okay-ish units (and definitely several underwhelming ones), pre-levelled, pre-DLinked.
I'm assuming here that you started playing before the client version update, in 2019. The update that introduced the Forward Basecamp. Because if you started playing after that, you'd have already completed this quest chain. It is almost literally "play the story in order".
Anyway, look at your quest menu. The second tab is where the "Get gold medal at stage X" kind of missions are. Among them you'll find one quest for a low-level mission, the reward should be something like 5 Combat Reports. There might be two such quests, I think Normal and Extra stages are tracked separately. Anyway, do it, and the next quest will open up. You will have to either replay the whole story in order, or go back-and-forth to check which mission is next, because it skips a few here and there. So decide which is less annoying for you.
It's not necessary to accept the mission rewards to get the next quest, same as with concutive dailies. Also, among the rewards are half-levelled LWMMG, SPAS-12, and Colt SAA, if you somehow didn't get those earlier.
One more thing, unrelated.
Matsuda's guide suggests pairing M200 with a Carcano. The purple one drops from the event crates. Also, as you probably know, if there was ever a unit you dismissed and now want it back, it's possible throught their Index page.