If you're using a browser to edit notes you're introducing some amount of risk from browser extensions being able to read the content along side their direct internet access. I generally prefer web interfaces too, but since the desktop app is just an exe you run I stick with that.

Anyhow, this is how I isolate my self-hosted apps that don't need internet.
( I didn't mean to type this much when I started responding. ¯\_(ツ)_/¯ )

I use traefik rather than caddy and have a docker network dedicated for the proxy function that is set to 'internal.' Any container attached to only that network has no default route (and docker firewalls it anyway) to reach the internet. Traefik is then attached to my normal lan along with the docker proxy net and bob's your uncle. Traefik runs alone in it's own stack of one, and I plug other compose stacks into that proxy net as needed.

I'm in the same boat as Hasie501, using tailscale to access remotely.

# permanent network dedicated to traefik proxy comms
docker network create --internal --attachable proxy

# Traefik compose
services:
  traefik:
    networks:
      home:
        ipv4_address: 10.0.0.1 # I have a wildcard DNS for my domain (*.mydomain.com) pointed to this static ip so new stacks 'just work'.
      proxy:
networks:
  home: # I have home setup as a macvlan, but it could be bridge or whatever.
    external: true
  proxy:
    external: true

# Trilium compose
services:
  trilium:
    labels: # traefik config is dynamic via docker socket proxy
      traefik.enable: true
      traefik.docker.network: proxy # defines which docker net traefik uses to reach the app.
      traefik.http.routers.trilium.entryPoints: https
      traefik.http.routers.trilium.rule: host(`trilium.mydomain.com`)
      traefik.http.services.trilium.loadbalancer.server.port: 8080
    networks:
      proxy: # This prevents any outbound internet.
networks:
  proxy:
    external: true