Some older homes don’t have insulation in the walls. They didn’t think of doing it back then because energy was relatively super dirt cheap. Without insulation in the walls, there’s not much to do as a renter. Whatever R Value you get out of the interior insulation methods you’ve been using won’t compare to the effectiveness of sealing all gaps and having insulation in the walls. Never work on your landlords place for free.

No real suggestions, sorry, I live in a cold house too. I invested in some comfy slippers and a warm baggy hoodie and they make a world of difference for me.