Personal Pet Question: Is it time to re-home? It might just break my heart.

Backstory: I had a heart cat of 15 years that passed away due to end stage renal, he was a managed level 2 until he blocked twice in a week and tanked what was left of his kidney function. A few months after his passing, I made the mistake of adopting a dopple ganger for him, who has been nothing but a menace in my life for 2.5 years (although I do love him). Two months into owning him, the single kitten syndrome was so unmanageable that we adopted a second kitten. They immediately bonded and while both of them have their quirks, I can at least get a full night's sleep. Fast forward to Sept 2024, I'm on my way home from a late shift, it's about 1am. I've never been hit with the distribution system IRL, but a little black cat darted out under my car and, managing to avoid flattening her, I took an hour to coax her into my coat and I took her home.

Current situation: I kept her mostly separate from the other two for about a month except for accidents/supervised sniffs. Right now, it's about 5 months later and they still don't really get along. She's a lot more confident than she used to be and will go all over the house and rarely hides, but she still doesn't want to engage with the others very much. She wants to play, but they overwhelm her and she will growl and hiss and then run. Sometimes, my male will give chase because I truly think he doesn't get it (he never has). When everyone's calm, they will all coexist great. They all sleep on my bed at night and there are no problems. I do worry about the continued stress of the others (especially my 16lb male) "picking on" her. It's probably not what it is, but it's what it feels like because she's still unhappy with the interactions.

Dilemma: She's a heart cat for me. I've bonded with her in a way I haven't with the other two (who I still love dearly) but I'm trying really hard not to be selfish. If this is something that seems like it will get better with more time, I'm happy to still keep engaging and separating them as needed to cool off. But if this is what it's going to be for her for good, where they just won't resolve the issue, then I will do what's best for her and for my current babies.

Alternative: I have also considered getting her a buddy. My original male/female have bonded hard and can never be separated and I'm wondering if a third wheel is uncomfortable for them in some way. If the new girl had a friend of her own, who also served as a buffer between her and the other two, is that a good idea? I have the space, the means, and the attention for another, but I don't want to end up rehoming TWO cats (which is why I was thinking foster-to-adopt, or just fostering).

TLDR; I found a third cat who has become my heart cat, but in the end it may be better to rehome her since she and my current two (bonded pair) keep spatting after 5 months (no fights, just spats). Do I give it more time, rehome, or try to get her her own bonding buddy?