I have little knowledge of OSPF but in real life scenario when multiple ospf area is needed
Would OSPF multiple areas be effective? Probably. Are they necessary? Hard to say. Again, much depends on actual topology and hardware being used. For instance, is there much of a topology at the HQ or branches? What's the link bandwidth, etc.?
A not uncommon OSPF design might be to make each site it's own area, which allows route filtering. However, for 30 spokes, one area might work fine too. Of course, you place mutiple sites within the same area to decrease the SPF demand per area without the need for numerous areas.