Sure, happy to throw out a few options. I think the idea of the flare stack is a good one, I would probably say keep that as a "fallback" so player can void byproducts if they don't want to deal with it and just want the oil.
If player does want to get some viability from byproducts there's two routes I can see.
1) Go route of Space Exploration's core chunk miner. Each surface has a unique chunk item that refines to that planet's unique resources at a set ratio. Byproduct for the geothermal vents could be unique for each planet, similar to what you've done recently with vents for the space age planets. Byproduct could give trace minerals requiring processing in chemical plant. I know you have your own deep core mining mod (I prefer to run that one than space exploration these days) so there might be an option to integrate across if you'd like to go that route
2) Other option would be to take inspiration from fulgora and set up an item similar to scrap that could be processed into a number of different basic resources. Obviously this would make sense to be a liquid but I know the coding for fluid recycling is not straightforward so a solid item may again work easier here. Processing would work the same as scrap with % chance for output, again possibly matching the planet's resource makeup.
Obviously a lot of balance to think about but those would just be two options to start with.
