A proper plastic and rubber setup would of course take the diluted packaged fuel route. And as that is a solid you could sink any excess there if any was converted to turbofuel. So that would make it geothermally safe.

That is not a problem in game do to this reasons.

1 - Plastic/Rubber production on late game does not produce extra fuel or any other liquid if you go through alternate recipes (you don't even produce cannisters on the chain, you supply like a container of them and then they are always being recicled through the system)

2 - Turbofuel/Fuel have an extra Polymer Resin that can go directly to sink or used to produce plastic/rubber/fabric

3- On Tier 5/6 you don't mix the lines (but to be fair if you're on termal you should be on tier 7)... on others you can do it but is not efficient and there is no reason to do it.