The .10 or .20 just advises Docker to create that specific Subinterface automatically. In my example ip link will show new interfaces called br0.10 and br0.20 after creating the macvlan networks for VLAN IDs 10 and 20. You do not need to adjust your Netplan config when doing it like that. I would even assume that you are not allowed to define VLAN ID 10 and 20 in that particular case also in Netplan. I would expect that this will cause issues. Also see https://docs.docker.com/engine/network/drivers/macvlan/ in the 802.1Q trunk bridge mode section.

There are probably multiple ways to do all of this, but this is how I did it and it works for me since a few years without touching it again. All VLANs are separated from each other and no VLAN has access to the LAN side. Everything is forced to go through tagged VLANs via the switch to the Firewall, where I then create rules to allow / deny traffic from / to all my networks and the Internet.

For me, this setup is very simple to re-implement should my Host go down. No special configuration in Netplan is needed. Only create the Docker Networks and start up my stacks again.

I can’t see your full setup / config from here, but a) you are not overengineering that. Using VLANs to segment networks is a very good practice. And although Docker (nor Podman) allow macvlan when running rootless, my gutfeeling tells me that segmenting my network takes priority over running rootless, because I think that attack vectors by traversing networks are much more common that breaking out of a container into the host. But this is just my gutfeeling. b) I think I run here what you want to achieve, so I try to explain what I did.

My Setup is similar to yours. OPNsense (OpenWRT before that), a Switch that is capable of VLAN and a Ubuntu Server with a single NIC that hosts all the Compose stacks.

1. You already configured your VLANs in OPNsense, so I will just mention that I created mine via Interface -> Devices -> VLAN on the LAN Interface of my OPNsense and then used the Assignments to finally make them available. On the OPNSense each one gets a static IP from the respective Network I defined for the VLAN.
2. On the Docker Host, in Netplan I configured the single NIC I have as a Bridge. I cannot remember if that was necessary or if I just planned ahead, should I add a 2nd NIC later on, to prevent that I need to reconfigure the whole networking again. Of course that Bridge sits in my LAN and the Netplan Config looks like this:

network:
  ethernets:
    eno1:
      dhcp4: no
  version: 2
  bridges:
    br0:
      addresses:
      - 192.x.x.3/24
      nameservers:
        addresses:
        - 192.x.x.x
        search:
        - my.lan
        - local
      routes:
      - to: default
        via: 192.x.x.1
      interfaces:
        - eno1

3. Now that the Docker Containers can use the VLANs, I had to create Docker Networks as macvlan like this:

docker network create -d macvlan --subnet=192.x.10.0/24 --gateway=192.x.10.1 -o parent=br0.10 vlan10
docker network create -d macvlan --subnet=192.x.20.0/24 --gateway=192.x.20.1 -o parent=br0.20 vlan20

4. Now for a Container to make use of those Networks, you have to define them as External in the Compose Stack like this:

services:
  my-service:
    image: blah
    ...
    networks:
      vlan10:

networks:
  vlan10:
    name: vlan10
    external: true

In 4. you have the option to not define an ipv4_address in the networks section. Then Docker will just pick its own addresses when the containers start. Letting OPNsense assign IP addresses dynamically in such a VLAN is something that did not work for me. So either you let Docker pick the IPs when starting a stack, or you define your IP addresses in the stack. If you do the latter, you have to do it for every stack that ever joins that VLAN, otherwise Docker might pick an IP that you already assigned manually and that stack will not start.

I also wanted to have some services running directly in the LAN via Docker. This setup is a bit more involved and requires you to create a SHIM Network, otherwise the Docker Host itself will not be capable of accessing Containers running in the LAN Network. This was the case for my Pi-Hole for example, that I wanted to have an IP in my LAN Network and had to be reachable by the Docker Host itself too. There is a very good post about Macvlan and SHIM Networks in this blog: https://blog.oddbit.com/post/2018-03-12-using-docker-macvlan-networks/

I hope this helps. Do not give up. Segmenting your Networks is important, especially if you plan to publish some services over the Internet.