Given the amount of effort I had to go to to "literally" bridge a smartphone to my network to give myself Internet access when my fibre connection went down, do you think this is slightly incorrect?

I'm assuming what you mean is that smartphones may be connecting to your internal network and bringing malware with them.

That said, the corporate networks I've seen have a separate network for phones/laptops and you need to VPN in if you want other access.

You are correct that activating both networks at the same time is hard, but what you have is a device which is traveling between untrusted and trusted networks. Assuming a compromised device anything is possible.