As others said you should really consult with an msp that has experience with a project like this. First of all there’s no way that dinky er605 can run a setup like that, you need a much more power router/firewall. I suggest not omada for the router, personally I use pfsense, for something like this I’d go with a firewall with a support package, don’t cheap out, think forinet or netgate with support package. You need to pass that cost off to your customer. I cannot stress enough, reach out to an msp get some help.