I assume that people in the gym will most often be facing the mirror?  If so, then you have them in the right location.  However, there isn’t really a need for the playbar here, unless there is also a TV on the wall that you want the playbar to play audio for.  I’d also be concerned if play:5 is right speakers for this room depending on how it’s going to be used.  Are you going to use the gym for classes with an instructor?  You would not be able to use play:5s as a PA system, and if the music is loud, people may have trouble hearing the instructor at the front of the room.    Even then, I might opt to go with in-ceiling speakers, or 4 Sonos Ones in the corners, in order to spread the audio more evenly through the space.

There is indeed a TV on the Wall above the mirrors on the left side, with a Google Chromecast in it, where people can stream their music videos, motivational videos or gym instructions/classes to. Sorry I forgot to mention that.

The other conclusion you’re making is also right, most people will be facing the mirrors most of the time but the could also use the machines spread over the room. There will be no specific classes with an instructor there so hearing other people won’t be of concern.

Overall the most important thing is the music in the room sounds the best at any given time (we mostly listen to hard dance music with a lot of bass in it)

Ok, when playing TV audio, you’re play:5 won’t be able to play in sync, you’ll want those to be off at those times.    I think the ideal setup for this would be 4 ceiling speakers powered by a Sonos Amp (which can connect to your TV), but since you already have the play:5s and playbar, you certainly could give this setup a go and see how you like it.