In Rogue Trader I feel really comfortable with switching out party members in and out of the group. It could be that the issue is not the slow advancement of the second archetype, but that someone from your party is the weak link and you need to replace so that the whole crew works better together. At least that was the case for me. Either way I'm interested in seeing how you progress with your current retinue and if you end up switching some of them for the later stages of the game.
If I remember correctly he's playing a melee blade dancer/pyro-psyker and Argenta is using flame weapons so that everything on his screen is burning as quickly as possible.So, Argenta has been heavily nerfed unfortunately, I kept her around in my last playthrough when the DLC came out and was basically useless, what is she using? Las?
But I frankly dunno how you may be in the situation of barely making it with the Janus end quest fight, your party seems allright (Yrliet is very very useful, but I understand the roleplay reasons behind getting rid of her)...unless they buffed the fight recently.
Did you make Cassia and Jae grand strategists?
Anyway, as you said it is more fun this way.