i also have very low porosity hair and for me some products with silicones are a godsend. however, it really depends on the formulation, some with dimethicone give me that dry-plastic feeling you're describing (i used to avoid them for a long time for this reason) but others that don't really differ that much in the ingredients give me lasting conditioning and make my hair feel alive. the rountine might also play a part, as they should ideally be sealing moisture in your hair and not preventing it from getting there - i think good leave in conditioners might be the "best tolerated" type of silicone product for this reason.
but yeah, you can't really count on most shampoos to wash them out completely even though they have sulfates. if you still wanted to experiment, there's silicones that are soluble in water (most of them have PEG in the name) and you can wash them out even with a sulfate free shampoo or co-washing so the buildup is out of the question, i have great results with them. a notable one from the "traditional" ones is amodimethicone, which latches to damaged parts of hair but repels itself, so it forms a layer on the hair without giving you buildup.
here are two great related articles from labmuffin,
one about silicones in general and
one about amodimethicone
here's also an useful but quite overwhelming list of
water soluble silicones, if a products has only those and those that evaporate, you don't have to use sulfates
Bookmarks