I have a homelab that I'll be installing home assistant to on a new virtual machine and want to make sure I made a network where devices on it cannot access the internet but devices on the network can access it and this is my first time messing with firewall settings. This new network would be under the IP range that starts at 192.168.5.1 instead of the default one for example. Also I already have Wireguard to connect to my home network normally but would I be able to connect to servers on this new ip range still even if it's a different network or will I need a new wireguard instance for this new network?
What you are asking can be achieved.
Which version of OPNsense are you running?
Your block rule appears fine but could be refined by using a negated RFC-1918 network alias as the destination. If you enable logging of this rule you will see when it is used.
You may have other rules which are affecting the flow of traffic in to this interface..
Please provide more information about your set up including other rules you have for this interface as well as any floating rules applied to it.
Rules for WireGuard will need to be created to access the "mediaserver" network or just the servers IP addresses, either for your existing WireGuard instance or with a new instance which only has access to that network, if you prefer.
You may want to include IPv4 Null Routes in OPNsense - see https://forum.opnsense.org/index.php?topic=50678.msg259031#msg259031