Is there any reason not to use the untagged VLAN?

I'm new to vlans and I'm looking to buy a router that supports them so I can use my old PC as a minecraft server for my friends.

Currently my home network has a "normal" network most devices are on and a separate mesh network that only a smart TV and mini computer acting as a network storage is on because they are on the far side of the house.

I would like to have a vlan for both of those along with a third vlan for the server. The problem is that the mesh network uses devices that do not support vlans. It seems that the only option to put the mesh network on a vlan without buying new hardware is to use an untagged vlan. I'm having trouble understanding the security implications of doing this while hosting a server. If my server were to be infected, would it be possible for the server to directly communicate with or discover devices on an untagged vlan?