No game this season is a gimme, you either play the long game and rest players now whilst we are in a relatively/fairly secure position with an eye on reducing chances of late in the season injury/fatigue AND to ensure the back up squad players get some actual game time at the highest level.
Or you try cement the LL trophy, win every game all the way up to and including the GF (which obviously is the ultimate goal CC aside) and hope that any minor injuries/niggles don't turn into something more or that players get burnout and perform worse come the crucial games.
Me, I'd rest players now, not all at once obviously but 2 players against Cas, 2 different players the following week. Trust to the squad players and the confidence within the team as a whole AND your coaching and tactics.

Play Carlos at Centre?

Again tough choices at centre and wing.

FWIW I'd play Pryce at stand off, not sure about the backs though

Talanoa, Fonua, Yeaman, Michaels IMO.

I honestly don't know!.
Yeamans defense could be needed against a physical Catalans team, but Carlos needs to be in somewhere.
Could Pryce be used off the bench again?.

I would play Pryce at stand off then a straight choice between Yeaman and Carlos at left centre. Tuli and Micheals on the wing.

I'd play pryce against his former club and yeaman needs to be in the centres as his defence is better than carlos. Then a decision on whether to carry a back on the bench but personally I'd go with 4 forwards