Not a good idea. This idea of a guest network is that it has access to the internet but not to the local network or other devices on the guest network.
An EdgeRouter X is incredibly cheap for the power it has. It requires some knowledge to configure though, there aren’t a lot of GUIs. A Ubiquiti AP AC Lite might be within your budget too, if you run the Unifi software on an existing server.
Sounds like a terrible idea. The wifi coverage will be awful and most cellular networks use CGNAT complicating remote access.
Viable idea but… unless you are quite familiar with VLANs you will probably end up making your network less secure.
I understand not all IoT devices would/could be easily configured to go SSL (we are talking about local SSL within LAN, right?) So if SSL is not 100% feasible, and VLAN introduces headaches, what other options do we have?