I’m a bit late to the party. I was a network admin in another life and believe I can help out. I’ve skimmed existing conversations but not fully poured over them.
That said: I’d print the routing tables for the switches and firewall and review them. Considering firewall rules is sensible but it seems you’ve already done quite a bit of investigation there. It sounds like there is some vlan configuration missing. Like one of the switches doesn’t have the vlan interface defined or named in its vlan database.
Review the routing tables and confirm who owns each vlan and what those networks are aware of. Also toggle your logging options on so you can review them and test with more info than pass/fail.
It very well may be a OPNsense firewall knowledge thing but I’m not convinced. If you’re still working on this tomorrow I’d be happy to help root-cause the problem and fill in the gaps between RTFM and old YouTube videos. :)
Not that I have any say in the matter but I think this kind of question is fine for the selfhost community. It’s still in the spirit of “how to do xyz with my own hardware / less dependence on a third party”.
The other suggestions have covered my own. I did want to add that good resources for finding these kinds of tools are often called Awesome lists. Awesome self-hosted. Awesome CLI. Awesome social media. You get it. I’m not sure where the trend started but I stumbled on them after getting fed up with the likes of Slashdot and AlternativeTo.
They’re usually a community-maintained git repo on one of the major platforms. This list has quite a few hits for “YouTube”, for instance: https://github.com/krzemienski/awesome-video