Okay, I'm going to spread my Hiitomo agenda from Twitter to here on Tumblr because, please hear me out, they're cute and go on omurice dates and practice together.
I think their dynamic would be the boke tsukkomi dynamic without them realizing, as well as mutual respect (with some jealousy from Tomoya's end that dissipates over time), with Hiiro seeing Tomoya as someone that's both vital to the others around him despite his normalcy and seeing him as a reliable person.
In an FS 3* for Tomoya (translated by nia_narqissa on Twitter), Hiiro outright calls Tomoya to be like air: "Not visible in the eye, tasteless, odorless, that's why unnoticed." The next line after that reads: "But without air, living being will die. A presence everyone needs." Which basically says that Hiiro believes Tomoya is someone that may not be seen as special, but without him, others may not succeed, and that he's needed by everyone. Tomoya jokes around with Hiiro but is happy that Hiiro uses his statement in a positive way, where the negative of being unnoticeable is replaced with the positive of being needed.
Also, Tomoya kind of guides Hiiro around and Hiiro sees Tomoya to be a different good from others. While Hiiro's good comes with talent, Tomoya's comes with care and reliability for others, like a safe space. Tomoya helps Hiiro learn more about things, like how he offers advice for Hiiro and tells him what he finds delicious and sometimes acting like a Senpai (though, technically, he is one in a sense of experience as an idol) during UNDEAD's MDM live.
TLDR: Please consider Hiitomo with their dynamic of mutual respect, advice giver x advice taker, and normal with issues about it x talented who finds the other's normalcy to be a good and important trait.