[comex]

To further clarify, running an internal DNS server doesn't require a MitM, as the DNS server address for a network is generally supplied as part of DHCP. (There is one reason you might want to do a MitM, but I really don't recommend it. Namely, some people change their settings to ignore the DHCP-supplied DNS server and hardcode an address, e.g. 8.8.8.8, which they would fail to reach if the network isn't connected to the Internet. In theory you could work around the issue by redirecting such traffic to your own DNS server.)

As for longest certs, the CA/Browser Forum Baseline Requirements (which all CAs have to follow) specify a maximum validity period of 825 days, or a little over 2 years. You should be able to find CAs offering certs with that period. (Why such a specific number? I have no idea.)

[shkkmo]

It is true, instead of MITMing the HTTP request, you can MITM the DNS request. The issue then is that you need to distribute and configure the private key and certificate on all the static file servers rather than just on the router