OK, then assuming that you already discarded to run 2 cables and others in this thread provided a very good guide of how to do vlan, I can only recommend to setup some aggregation channels in your opnsense box and some switches to at least mitigated the performance hit.

Not now, of course, first you need yiur setup working

Then I really recommend you not to play with vlan and use different eth ports as different segments.

The performance will be significant better this way

Just for me to fully understand your setup.

How many eth ports do you have in your opnsense box?

Try to find a rev3, the cpu upgrade really is really worthy

Which hw revision?

If it is a ver3 it is the same I have, good for FW and red services, you can make complex setups.

It is a bit short on cpu for ips systems(suricata and zenarmor) , but it is able to do dns filtering via adguard or unbound.

100€ sounds good to me if it comes with AP, people here are happy with that brand

Good luck

Take a sophos second hand FW.

Intel nic, low power consumption processors and full opnsense support.

Go at least for 4gb ram and the most powerful processor you can safely get. It will come with a lot of eth ports too on top.

And it will cost close to 100€, probably less if you struck a good deal

The only thing I can tell is that it is totally worthless.

Because you can not have an uga ipv6, then obviously you will have an ula served via dhcpv6 with ipv4 local address (double stack).

And in this point you will realize that most computers implementation select which ip address to use following prio: ipv6 uga -> ipv4 -> ipv6 ula

So even if you do all the things right, your clients will not use it because ipv4 is there and you cannot deselect it because I assume you still want connectivity

Funny, right? :)