I think, personally, Pink Diamond had the idea to retrieve Spinel after the war, which is why she wanted her to stay there, and not move, so she could find her, but then, as we all know, the Galaxy Warp was destroyed, and no one had the tools to fix it.
Honestly, I'm with you there. I mean, I think Pink just kind of... flippantly left Spinel there because at that time she was pretty much full-on Diamond Authority-minded and probably didn't really see Spinel as a person, but rather a toy. She just discarded her in the garden like an old toy and at the time probably didn't give a second thought on whether she'd ever go back.
But I think Spinel was probably one of Rose's many MANY regrets from when she was Pink Diamond. And I think Rose fully intended on rectifying those regrets. I think she intended on dismantling the Zoo and intended on retrieving Spinel. Heck, she probably intended to spread the Crystal Gem way of life to other planets eventually
But everything changed at the end of the Rebellion. They drove Homeworld off Earth at the cost of the Crystal Gems being absolutely obliterated. Every single Gem Rose knew and cared for was brutally corrupted, except for the two who just happened to be literally standing right next to her. The Crystal Gems ranks were reduced to three Gems in a blink, leaving them with little to no resources. Any other grand plan they intended to do was no longer an option without the gear and Gems to do it
But even beyond the lack of resources, the toll to morale would've been devastating. This big rebellion she led that was supposed to fix everything, free everyone, create a happily ever after backfired so hard and everyone, every single Gem affect by it, it was because of her, because of her actions. And she has healing powers but they do nothing to help. Can you imagine the guilt? The absolute feeling of defeat? I can't imagine how hard it would be to fathom mounting another attack or move on Homeworld to dismantle the Zoo or retrieve Spinel when the last move against Homeworld was that devastating.
And, like, the Crystal Gems' time on Earth wasn't a pleasure cruise. Sure, they lived life and enjoyed things, but they were also working hard to retrieve and contain the corrupted Gems, a task they were still doing even when Steven was around, and trying to find a cure, not to mention doing all this in a quiet world that was once full of Crystal Gems, a ghost town of a Gem refuge that never was. I imagine it would be hard to face that every day and not lose the will to try for another big plan
So, yeah, I think Pink discarded Spinel very coldly at the time, but I also think that, years later as Rose Quartz she wasn't just kicking back relaxing without a care of the hurt she'd done. I think she was very painfully aware of it (honestly, Rose always seemed to kind of hate herself in flashback episodes) and I think it just got filed away in her long, long, long list of regrets out of necessity/a lack of feeling she could do anything about it