If you’re using OPNsense I recommend you install and enable Avahi to help route discovery traffic, and I would double check multicast is functioning on your wifi.

IIRC with home assistant there are some commands you can run to make it aware of your VLANs… have a look at this

+1 for shelly, a few options that could work.

is multicast DNS enabled and available in that VLAN?

have you set up the network for HA using the

ha network 

commands?

have a look at the Shelly H&T (and the new G3 version). They are cost effective battery or mains powered temperature sensors that include both bluetooth and wifi. They work locally with no hub required but can also report to the cloud where alerting can be configured.