Basically, you just need to get the commercial enterprise tradition in the mercantile thing, which gives you 1 merchat for commerce zones/trade districts.
that should be your priority tradition, after that, you build as many commerce zones and trade districts as possible.
the void dweller start helps with that.
and you can build them (orbital habitats) on planets/whatever else generating trade value to be able to build trade districts on them.
and then you just get as many merchants as possible, and also form a trade federation and use the thing where it gives you consumer goods, unity and enegry credits for trade value.
You don't want to employ the clerks from the buildings though.
Clerks are like, horrible.
And btw the lithiod species helps you still settle planets early on even though you're a void dweller.
which is useful.
this build used to be incredibly op pre 3.4 but it's still pretty good.
and very fun.
But honestly megacorps are pretty good in general, just make sure you pick the right stuff (which there is a lot of, like for example being some degree of xenophile for the extra trade value) and you'll be good to go unless you're playing on the highest difficulty.