Ok, maybe I’m exaggerating on the “every episode” but you’re still going to need at least three actors to play the main character, plus intermediate appearances as they transform which lasts an episode or more. It’s definitely a significant additional problem that you just don’t have to deal with if you start with one of the other books.
The other challenge if you are thinking about doing multiple books and you start with Phlebas is that the Culture are the baddies. You spend a (most) of a series presenting them as such and then flip.

Someone made the point previously about Disney owning Muppets now and doing live action adaptations of all the animated films when they have the opportunity to do muppet versions instead.

Although I would absolutely want more farscape.

TV limited series adaptations of The Culture novels. Amazon obviously made a start before giving up but they picked absolutely the wrong one to start with.
Yes Consider Phlebus is the first chronologically but a TV series where you have to replace the main character every episode does not work when you are already trying to adapt a very difficult IP. Player of Games is where you want to begin. Clear storyline, no tricky time/universe/world jumps. Spends most of the time in an Earth-esque civilization. If you can successfully adapt PoG then you can try something more challenging like Phlebus or Use of Weapons.

How did the first astronauts get inside?