Lets do something with all of these Credits

The most crystal efficient way to improve your mods is to buy mods from the mod store, and constantly refresh it. 

Unlike other stores, refreshing the mod store only costs 15 credits. This means you get as many or more "possibles" for 15 credits as you do for 50 if you refreshed mod energy and did the challenge. 

But it's even better than that, bc a HUGE proportion of the Mod Challenges mods are lowest-quality and hardly any are highest quality. On average you'll get one good mod on 2/3 or 3/4 mod store refreshes, where if you go the Mod Challenge route you get less for more crystals. You burn through credits while avoiding running short on Mk5 slicing materials (since you're only buying Purple and Gold mods and thus need to do much less slicing at the Mk5 tier). 

Since you and everyone else have more credits than good mods or slicing materials, there's nothing but win here.