DHCP Server

Using the ISC DHCP server is the best and easier choice.

You should disable the ISC DHCP client as it is deprecated (and you already have the Gentoo's DHCPCD client anyway), so create the file /etc/portage/package.use/dhcp:

dhcp
net-misc/dhcp -client

then just emerge it: 

emerge net-misc/dhcp

Edit the DHCP configuration file /etc/dhcp/dhcpd.conf:

dhcpd.conf
authoritative;
subnet 192.168.0.0 netmask 255.255.255.0 {
     # disable DHCP on ISP#1 upstream network
}
subnet 192.168.1.0 netmask 255.255.255.0 {
     # disable DHCP on ISP#2 upstream network
}
subnet 10.0.0.0 netmask 255.255.255.0 {
  range 10.0.0.100 10.0.0.250;
  option domain-name-servers 10.0.0.1;
  option domain-name "mydomain.com";
  option routers 10.0.0.1;
}
host fixed-ip-pc {
    hardware ethernet 12:34:56:78:9a:bc;
    fixed-address 10.0.0.95;
}

Here i assign a pool od dynamic IP addresses (from 100 to 254) on the 10.0.0.0 subnet. Addresses under 100 can be used for static assignments. For example, i use static IPs for all my OpenWRT Access Points and wired security cameras, and dynamic for all other devices. I have also defined my two ISP's subnets to explicitly disable DHCP assignments on them, which is not needed and might be an attack vector.

This config will also ensure that all devices will use the home server both as DNS server and gateway.

This will not work for devices that use hard-coded DNS servers (like Fire Sticks and Google Chromecasts…) but there is a workaround for those too, and i will show you on Routing on the Home Server.

Unfortunately, again, the mess that DNS over HTTPS is (DoH), make it impossible to set your own DNS DoH server over DHCP. This includes many Android devices, that use DoH by default and will always use Google resolvers unless you specify one manually for every network you connect to.

Well, this is almost all. Start dhcp service and make it start on boot: 

rc-update add dhcp default
/etc/init.t/dhcp start

Now you can connect your devices to the home network and they will get an IP address and a full network configuration to go with it.