let's talk about having no feelings
So, Flowey. No feelings. Not a single one. It's very weird to describe what that's like, because usually something is going on in your heart that is causing a feeling, even if it's a mild anticipation or relaxation or something. Flowey's lack of a soul means he doesn't have any background noise in his being to supplement what other people feel. The overwhelming sense of compassion he felt with all of the Underground's souls convinced him, despite years of solitude and desperation, to give them back to their owners.

The point is, when he gets the two feelings that make up his projected personality in the original game (pettiness, antagonism), he's gonna have a ball with them. Having one or two feelings after having zero for a long, long time and a brief reprieve at the end of the True Pacifist ending is going to be a lot for Flowey. The entire process of going through important threads that gives him his regains is going to send him through several loops and then some. For instance, being able to process guilt while not having access to reflection will be hard for him. In others, having tender feelings but not love will be strange because it's ~so close and yet so far~.

There might be a point where regaining grief might be a happy feeling for him. Same with anger-- real anger, towards something that deserves it and not what he's taking it out on to try and feel something. The more he veers towards positive emotions, the more Asriel-like he'll get. If he veers more towards negative in the beginning, the more Floweyish he'll stay for a bit. As seen in the game, what he does possess of feelings seem to relate to survival instincts.

I am willing to go in whatever order it takes for him to get back all of his feelings, positive or negative, and see how he deals with the roulette he spins each time!