I honestly can’t imagine why an all-time great who has a super cushy job talking football with his brother would want the stress of head coaching.

I think this is all in your head. The creative on the show didn't change between seasons and the quality is pretty consistent.

I would guess it's some combination of wanting to keep the cast salaries lower and wanting to make sure that the cast they got wouldn't have a ton of scheduling conflicts as they'd be required to film five seasons in New Zealand. It's also possible that Amazon Prime, as a newer studio, has a tougher time getting big-name actors to join their shows than established studios like WB and Disney do without paying out the nose for it.

Will Poulter was originally supposed to be in the show and had to drop out because of scheduling conflicts. Guessing any known commodity would have had the same issue.

FWIW I think the cast is solid and not at all a problem for the show.

It was entertaining throughout but had some flaws. It was often jarring how much time passed between episodes and some of the bigger moments didn't land as well because characters or character motivations were barely established.

S2 shouldn't have any of these issues though as the showrunner has said the time jumps are done and the show will focus on the Dance fully now. So definitely looking forward to the next season.