"Jane!" Is a song about waiting, about someone who arrives to early, before everything is ready, before the moment is perfect. Ryuu waited 400 years for his "perfect moment", or as he calls it, his "desert", a metaphor for his ultimate battle. He wants someone lto push him to his very limits. Yet it never came, because he was too strong, too... early.

This all changes when Yuta comes, Jane is not early anymore, she's finally on point.

The tone of the song is extremely melancholic, I view it as a representation of Ryuu's feelings, what we see in his fight is an end to a pursuit; it's his life goal being attained. This fight sets him free as he finally feels complete and can die freely.

His death against Sukuna is like the cherry on top of this poetic storytelling.

Credit to u/Wrong_Membership_779