I think saving crystals and buying mod packs when they appear in the store is the best way for mods. 1 and 2 dot mods aren't competitive in the long run, and even mod challenges rarely give mods with prefilled secondaries (which upgrade to higher values when levelled). Mod packs are full of these mods and if you save the crystals you spend in mod challenges for these packs the overall value is better in my opinion.
As a quick fix to stay in top Arena competition it is very good what you did, but honestly I wouldn't bother too much with the challenges. On top of crystals for challenges you also have the considerable trouble to farm, level and gear specific teams (except for health and tenacity mods but no one wants to farm tenacity mods anyway).