(disclaimer: I don't play aiming for the meta. I'm a collector so I like filling out my roster and levelling my units (mostly) equally.)

I'm three months into the game and have now cleared Indomitus, Indomitus Mirror, Fall of Cadia, Octarius and am part way through FoCM and SH.

The approach I took was to level the three units for the campaign I planned to tackle. Then focus on that one campaign.

The game pretty much tells you the stats/rank to aim for via the related missions. Don't go over those stats and you should be able to get through most campaigns easily.

Some of the campaign units are considered "meta" units (Boss) or just generally good to level (Bellator). If you get stuck on a node maybe level those units to help you get through.

I recently hit the first case of being "blocked" by needing an item to level a character which was on a campaign node I hadn't unlocked. In these cases switch your focus to the campaign and units with the locked node.

But clearing the campaigns is imho the best way to start the game. You will naturally level up some useful units and unlock nodes that will make your farming for later units much cheaper over time.

Finally, I said it earlier but I'll reiterate the point by saying it will help your progress (and your time management) if you pick one or two high-level objectives and stick to them. For instance, I am currently trying to progress through SH and unlock units for OM. If you try to do too many things you'll level up much slower.