Bret Devereaux

On the one hand, it's hard to imagine how the "picked thorakitai" of Ptolemaic Egypt (attested 197 BC) would have been all that different in appearance from legionary heavy infantry. But the Roman organizational scheme and adoption of gladius probably set apart the Daphne 5000

Yeah, the thing about the Daphne procession is that Polybius is explicit on the 'Romanness' of the picked 5000, which I think argues against reading their equipment as being understood as culturally Galatian.

Personally, I think this has to do with the fact that they were mailed infantry. We have a decent amount of evidence for Gallic mounted elites wearing mail, but unsurprisingly such an expensive armor seems to have not been used by the infantry.

Whereas I am quite confident that by the 160s BC, mail was the majority armor-type of the Roman heavy infantry.

What was the harder formation to emulate? The Roman cohort structure of heavy javelin/big shield/stabby stab - or - sarissa phalanx?

I'm with @DrMichaelJTayl1 - it's the legion. The necessary equipment is more expensive and you have to train and prepare for a greater degree of local decision making Sure a file-leader leads his file, but how many units in a phalanx you have individually maneuvering?

Alexander's pezhetairoi had 6 taxeis which might maneuver independently, plus the hypaspists. But assuming we buy the standard cascade attack of the legion (and I think we should), every maniple of a legion can maneuver separately.

So you have that triplex acies w/ potentially thirty separately maneuvering blocks of men per legion, doing substantially more complex maneuvers than the sarissa-phalanx was typically capable of. The number of guys you need who can 'drive' a unit around the field is much higher.