Entry Level Router/Firewall with WAN failover and Larger internal subnets on VE

Hello,

I worked at a datacenter as a SysAdmin for few years, but never really got heavy into the networking side. I have some basic knowledge.

I have a client that I am doing a cryptocurrency mining buildout for. I have want to put them behind a nice firewall/router with WAN failover and the ability to include a large internal subnet, say at least a /21 on a vlan, and pass that vlan through to multiple unmanaged switches.

I think I would prefer a router with a GUI option, maybe a newer mid-level Sonicwall, as opposed to an ASA with only command line, for the time being, until my networking knowledge grows a bit more. I might practice with some ASA command line in GNS3 in the interim.

What would be your hardware suggestion, and any suggestions in general?

Thanks!

PS: Does a TZ500 on the low end, support up to a /21 internal subnet on Vlan...what is it's subnet size limitation?