When looking at the current (3rd sphere) map, it seems that expansion is not an all-around movement, more like a growing extension somewhere in the Empire.
1st expansion: growing around T'au
2nd expansion: growing around T'au but more to the "south" of the Empire (if "south" means anything in 3D)
3rd expansion: probably launched by T'au, Vior'la, Sa'cea and Dal'yth to the "north".
So the Empire may have been launching new 4th and 5th Expansion fleet from other parts of the Empire (2nd Sphere septs and/or 1srt sphere Bork'an, Fal'shia, D'yanoi, ...) in other directions. These fleets could have been build alongside the 3rd, but this one was the first to be launched and the other were probably still being built/or in transit to their expansion rendez-vous point.
With a growing empire, the number of direction they can expand is growing too. So multiple and simulataneous expansion fleet are likely.
Plus, strategically, it is ridiculous to expand the Empire borders equally all-around, better launching your fleet to clusters of star systems to colonize quickly them. So the expansions "spheres" are probably spatially spherical only in their growth from their destination point.
The problem remain the travel speed. 4th is lost in the Warp, so they may be (or not) far away and/or splitted.
Until GW retcon our limited speed for the new spheres, Tau cannot spread far and quickly.
It is likely we have, or are on the verge to have, FTL travel: Bor'kan is an academic sept, Fal'shia made many innovations, Elsy'eir is creative, Vash'ya has skilled Air caste, ... Sa'cea, Viorla, are more military than engeener, so their fleet may not have the latest innovations (ideas spread too at the speed of light before FTL travel).
Even with FTL, recon flight and observation takes time until you launch a new expedition in unknwon areas.
3rd Sphere seems to be oriented toward clusters of imperium systems on the other side of Damocles, "discovered" when interrogating prisoners/turncoats from the 1st Crusade, so it can be planned quicker.
Problems concerning resources (raw material, people, and, the more important, energy) are minor if fleets have been being built from a few decades back in the past.
I expect new spheres to be launched in other direction than Damocles, assembled from other parts of the Empire that are more technically innovative than the current more military septs involved in the Damocles crusade and subsequent colonization in the retaliation move.
I want FTL travel, new battlesuit/vehicle designs, new auxiliaries, ...
And a real 3D map from GW.
I tried to do one in the past, but with little infos, only maps from 2 point of view and some fluff, it is hard to place correctly all the Tau Septs:
(The red arrow is the Tyranid invasion, not Damocles Crusade: its arrow would have been on the "top" of the empire)