At least on paper, the solution seems fairly simple to me: allow just one detachment per army at least up to 2k points.
Imho, the available set of detachments gives people great choice of army structures, and compensates for the fact that different indexes may have their best units concentrated in different FOC sections. However, any of the available detachments is more than enough to accommodate a full 2000 pts army, provided its units feature some reasonable variety of battlefield roles. IMHO this is much more reasonable and fair than restricting people to just battalions, but allowing multiples of those.
Maybe make it one "standard" detachment + one super-heavy auxiliary + one fortification.