best (temporal/spatial interpolation-capable) keyframe plugin for Final Cut Pro???

Do you have Motion?

It seems to me that the sort of animation you are considering would be easily done in Motion. There you have a great deal more flexibility and control over keyframes, not to mention behaviors, with which you can obtain a lot of results with less work than keyframes.

When developing templates, I always have Motion and Final Cut running at the same time.

You asked if it's dynamically linked — the answer is Yes and No, at least not in the way you're thinking about it.

Final Cut retains "revisions" as long as it's running. If you shut it down and restart it, then those "revisions" all revert to the last saved version of a template in Motion. I actually find this convenient because I can have a string of clips, each with a different saved version of a Motion project while I'm working on it.

The Yes answer is that while you're working in Motion and Save a "state", it is automatically updated and available in Final Cut... but it has to be added again into a project to see the changes.

As for it's power to create "keyframes":

In this project, I've designed a line that can be customized in Motion and Final Cut (with onscreen controls) that controls not only the Y rotation, but also the X rotation and Scale, all at the same time or with custom mixes of all three at the same time (so each one can be soloed as well):

The blue rectangle with the "path" line on it is a visual control base, it's width is Time and its vertical measure is Value. I have the OSCs turned off from view (not necessary to make my point). By default, the top/bottom edges represent either 180º rotation and +/- 100% scale, but these values can be "conditioned" with parameter scaling (which may or may not be published for Final Cut). The Effect Source ("drop zone") is scaled to 50% for this illustration and normally would probably be full screen with the option to scale it provided to the end user.

Playback looks like this:

Consider that this is a 15fps GIF.

This template should be published as an Effect so that the end user can Save As Effects Preset for any and all curve/path designs they create for future use.

I would estimate that there is at least equal power with Premiere/After Effects, albeit slightly different approaches.

I suspect it’s not likely to change. Perhaps you can do the animations you want in Motion.

I assume you’ve used feedback?