Do you really need plastic? I am sure you don't need rubber but one is equal to the other regarding ressources. You don't need a cockpit to leave atmosphere (it makes thing easier though)

You can get off into orbit by using hydrogen thruster which, afaik, have Cobalt as the most expensive ingot; everything else should be mineable by getting stone or the cheap ores.

Raiding unknown signals won't give you plastic but at least Lithium Cells and Aluminum plates, both can be used to speed up teching a bit/making your life easier.
But wait. Maybe you can disassemble parachutes into fabrics and disassemble that further into plastic!

I play with "normal" meteor storms and grabbed some sulfur from the impact spots.

Hope it helps, lemme know if it brought you forward.

quite nice idea with the meteor storms, especially when you have build an underground base.