KoosPol

@ampersandrew Also, if you need to use a managed switch make sure all VLANs go to all ports with only one PVID. Remove as many puzzles as possible.

@ampersandrew It's quite a lengthy post. Admittedly I only skimmed it. Apologies if you already provided the details.
If your firewall has sufficient ports you could hook your laptop directly onto it. If you're running linux, your laptop is 99% certain VLAN aware. In that case you could use dumb switches.

@ampersandrew You're battling two problems at the same time. First get rid of the managed switches (or disable 802.1Q). That will make you life a lot easier.