I wouldn't care if Peach and Lucifer got back together. They still care for one another and It's not out of the realm of possibility that they have a healthy relationship the 2nd time around. But he needs to work through using someone to make his ex jealous and she needs to work through her lingering obsession.
Suji getting with the person that helped create the idealized version of herself and is using her to get his ex's attention is not inherently better.