Instead of centrifugation recipes, the spoilage to carbon recipe could be changed to yield coal instead. Then the coal to carbon recipe could be unlocked on both vulcanus and gleba at the appropriate tech.
For early plastic on vulcanus, adding a small trickle of petroleum gas to the simple coal liquefaction recipe would do the trick. Like 5 gas per craft maybe.

I considered this sort of approach and others when I started designing this mod, and decided that I wanted to make as few changes as possible. And that what changes I make to make starting on a planet even possible should never be a better alternative than vanilla methods of achieving the same output. Another consideration was to not break existing designs, so changing an existing recipe to have a new output would break all simple coal liquefaction in an existing save, or in existing blueprints.

I want to leave that kind of thing to other mods (whether I make them or others do) that are designed specifically to make starting on a particular planet more enjoyable or interesting. The changes I have in this mod are only the bare minimum required to make it plausible to start on any of the vanilla planets, and also let you disable those changes if/when you install a mod that achieves that goal in a more interesting way.

I appreciate the thought though. You might consider your own mod that does exactly this! I'd love to see more people thinking about and working on the problem of starting on alternate planets. CodeGreen has done amazing work with solving this problem at the data phase, but I don't know of anyone else exploring the runtime phase solution as I have.

Makes sense, I didn't think about it that way. In that case, what do you think about the following ideas:
- for Gleba, unlock the coal synthesis recipe earlier, like with the bioflux processing tech
- for Vulcanus, unlock chem plants and heavy/light oil cracking recipes with calcite processing