Should have I read or watched other Idolish works before picking this up? Maybe. Does it excuse Before the Radiant Glory being absolutely uninteresting? Not at all.
This manga is quite short, even for a one volume only manga (150 pages, which is already thin and even then are 20 of those pages non-chapter extras), so don't expect much to happen story-wise. This is barely a character introduction, and even for this it doesn't offer much. Well, to call those characters "characters" may too be a bit of a reach.
There's something closely resembling a story in the last third of this manga, but it's honestly
ridiculous. It's about the band being booked for a charity concert which they want to cancel because it would give them "unfair" advantage over their rivals. They are choosing stupid rivalry over a god damn charity concert. And those are supposed to be the protagonist. The manga even dares to paint this in good light, like how they are good sportsman by not wanting to do it. Bruh it's a fucking charity concert that you are even already booked for, just suck it up and do it, how is this even a source of drama in the first place?
The art borderlines between average and acceptable, though where it fails is the character design. Literally every character maybe except for one member of the rival band (who is shown for like two pages) looks like a generic bishounen character with nothing to remember them by.
I'm sure fangirls will love it, and enjoying it because you like the franchise is fine, but if you stumbled upon this randomly like me, just don't expect anything of substance.