Let me try and answer the questions asked:
1/ The upstream router is a pfsense box that has other devices connected and runs a dhcp server
2/ I am not very experienced with openwrt but am more comfortable with its GUI interface
3/ as for where the networks should be separated, I'm open to suggestions as what would work best
As for me, I am a programmer with some basic, home-grown knowledge of networking.
Thank you for your help