Looking good, and understand what you mean about stuff not going very far. As always, looking forward to seeing this in the flesh.
Thinking about the shortage of suitable pieces vice the cost of GW scenery and the excess of stuff you don't generally need, one solution I toyed with a few years ago was to make a latex mould of the pieces and then resin cast as many as you want/need. I've made a few silicon moulds in the past and you'll be surprised how easy it is to do.
I'm not sure what the cost of plasticard is at the moment, but have you considered using foamboard for some of the structural sections? Saying that, is the base foamboard?
The other option might be to consider some stuff like this:
https://www.thingiverse.com/BiancaW/designs There are some 6mm Epic designs in the library but the beauty of printing is you can scale as appropriate. Happy to have a chat and help out if needed.