1) plutonium fuel cells can't take prod modules
2) the recipe only produces 1
3) they burn up very very fast, not sure how fast, maybe 30 seconds? certainly much faster than a vanilla uranium fuel cell as comparison
4) getting (enough) plutonium is hard/annoying and very slow
( 5) yes the reprocessing does take prod, but... per default it's 5 -> 1 plutonium if my math it correct so even with some prod it doesn't change a whole lot)

All that means that you basically can't reliably run the reactor. It took hours to get enough plutonium for a few cells which ran the reactor for a few minutes to be able to get a bit of lithium. And nothing more.
Thankfully getting power from it is kinda optional, solar is good enough to launch a rocket (even though you'll be constantly on low power).

But... at first you think getting the reactor is cool and a game changer, but instead I just run it for a short time to get a bit of lithium. Importing a fusion reactor feels worth it (long term, haven't been to Aquilo). Which is silly.
I guess getting the reactor to legendary would help a bit because it would waste less heat, but that's crazy.

In short: I just feel some balance is off there. The burn time could probably be doubled and it would still be hard to run it consistently at this point.
Personally - without doing any more math here and I'm not saying that's what should get changed, those are just some thoughts - I would 1) allow prod for the fuel cell recipe, 2) double the fuel cell recipe output from 1 to 2, 3) double the burn time in the reactor. Something like that.

Getting the reactor is hard enough, getting plutonium is annoying enough. The reactor should feel amazing and not just the means to get lithium (which you only need for lubricant for the electric engines for the rocket silo which is also a bit silly).

So I haven't tested your changes yet, but the 10X to 30X seems nice (was using belts already, so my build should immediately profit).
"Plutonium productivity" seems like an interesting idea, will also have to check that out.
I dislike losing the engine prod techs because 1) it possibly affects already existing builds and 2) it gives Cerys even less use for other planets.
While I first thought the engine prod was a bit weird I actually like it, Cerys itself profits because the repair packs need engines and in general the vanilla blue science profits as well (also bot production). I'd love to not lose those techs, why not just add the plutonium prod instead of replacing. (Also engine production is annoying to scale: slow to produce and neither foundry, nor EMP or any other special buildings work with it. The engine prod techs fill a perfect niche.)

The bug fix for heat pipe drain seems also interesting. I wonder how much that affected my experience. I plan to go back to Cerys soonish and maybe try to build it up a bit, especially with those changes.