What kind of routing issues would I have if the netmaker server would be on the same local network as the devices it will manage ?

totally doable.... the endpoints of peers should be set to the local lan ip rather than the public ip .... when calculating peer config.. if netmaker determines that the peer's public ip is the same as the nodes's public ip it will use the local ip as the endpoint instead.

Oh, that's nice, I assume on external clients like an iOS device/Android device I have to point it to the Static IP of the Netmaker server ?

the config for extclients will be grenerated with the correct endpoint

Also, I'm planning on running Netmaker + Wireguard on an Intel Atom N450, would it be a good solution for 3-4 clients ?

no idea, have not tried it on that hardware

Oh ye, I'll also have my own DNS, Adguard Home, will that affect anything ? Also, since I have no domains, I assume I'll use the static IP of the device which will run netmaker ?