When ZUN isn’t trapped by the confines of games, characters, or any of that jazz he really shines no matter what.
Even as the music in the games becomes more disputable, the CD’s have always stood their ground firmly and this has been shown time and time again, with basically every CD essentially reaffirming to everyone that ZUN has not lost this edge when it comes to making music.
Obviously the best case scenario is both games and other media have all the same level of extremely high quality music, though I do think some of your disappointment with UDoALG’s music is personal preference as many others love the themes, but at least we can say that ZUN still puts a lot of effort into the CD’s, though his tastes have changed with age and he probably puts less effort into the music overall, though he weirdly goes insanely hard on random things like fan album contributions or one off things like Independent Simulacrum so who knows.
tl;dr :ZUN is his own man and none of us have basically any say in what he does, if he wants to do something he does it because practically no one can tell him no, he’s the one making the music, we’re just listening to it and rearranging it-when touhou music doesn’t sound as good, you can probably bet ZUN just put less effort into it, because why the hell would he lose his ability to compose overtime? (Im not saying we shouldnt criticize touhou music when we dont like it, frankly just saying dont lose hope, zun will probably always put something out of nowhere and suprise us all again anyway)