Outside of the spine which is more or less set (Hynes, Moylan, Kennedy) and selections forced from injury but not entirely (e.g. Dykes over Metcalf/Miller) Fitz doesn't seem to have a rigid set pecking order with his players.
Looks to pick players based on merits of each game/opposition/gameplan required with no doubt some consideration thrown in around player fitness, how they trained ect.
Most coaches have their pets or favs that get selected no matter what or they want to keep team cohesion and combos by selecting the same copy paste 17 every week but with Fitz we see a variety of different forwards and backs can be picked one week and then someone else the next - varying in experience, age, body type, ect. Miller, Tolman, Fifita, Jesse C, Ikavalu, Hazleton, ect ect.
It's not like he is chopping and changing selections because of form or bad performances as most of our players have been reasonably solid and consistent so it's not like they are getting dropped for someone obviously better necessarily.
Maybe this is Fitz way of spreading the game time and experience around the team along with balancing out player workloads keeping older players more fresh for finals while his younger fringe players will have some confidence and games under their belt.
Can't say it isn't working as despite the team disruptions and changes we keep on winning and it's refreshing in regards to coaching and team selections from a fan's perspective. It makes sense to keep the spine stable and the same week to week (assuming fit and not suspended) and shuffle some pieces around that as required or when deemed beneficial/necessary.
Maybe Fitz does have a best/set 17 in his mind that if avaialble he would play each and every week but it doesn't quite seem that way especially when it comes to the forwards/bench and who comes in from a depth point of view.