Airi enjoys Hina's pain, but I think Airi enjoys the fact that Hina "endures" for Airi specifically more than anything else.
Definitely this. It's why she gets off on the blindfold thing. Airi loves watching Hina endure the mental and emotional suffering/suspense of not knowing if Airi abandoned her, but still sitting there and obeying her anyway. It's the way that Hina values Airi's commands over Hina's need to escape whatever pain she's going through
BDSM isn't about pain. It's about power and trust, whereby the sub surrenders all autonomy to a dom who has unfettered power. The dom derives pleasure from being permitted to do anything in their discretion without traumatizing, maiming or killing their partner. The sub derives pleasure from being pushed to their sensory and emotional limit, with a boost from their partners demonstrated willingness to surrender their superiority by not harming them too much.
In this relationship Hina has greater self confidence than Airi. The latter has opted for the high achiever route, sacrificing her autonomy for a role of suffocating self abnegation. After holding her breath and playing the perfect robot, she goes back to Hina and vents her frustration, a highly intimate privilege afforded to her waifu.
Their expression of love for each other takes an extreme form under dysfunctional circumstances (the extreme, coerced conformity of Japanese institutional environments) but the interplay between them is very healthy.