the game does not tell you that the new concentrate scales with dex and much more so than gungho with atk afaik.
if you go for the concentrate/dex build (I think 1 point of dex = 3 points concentrate damage* without the attribute bonus which goes straight through def. It’s basically a RO crit assa hehe. And you still get the crit damage bonus from dex.
With my full dex build at level 200 hexen dropper does like 3x as much damage with concentrate on…
Reason for going peltasta is afaik not the swashbuckeling but the guardian which gives you a not insignificant defense bonus for losing some atk (but you do not lose the concentrate bonus) but also 150 extra dodge (equivalent to 150 dex) so it is not bad at all with a full dex build. Fixes some of the survivability issues you have for not having a shield.
*not 100% sure here. Could just be 2 points.