That was also my question. A broader question is how to access services on the local network that are announced through local DNS? Like your router's web interface or any similar device.
Can you have split routing? Most queries go to our preferred DNSoverTLS endpoint, but some go to DNS53 on the local network.
This would also solve the captive portal if the host used to detect captive portals is always resolved locally.
I'm also a happy osmand+ user, but once i got lost in the middle of a field / bush. I'm sure there was a trail once, but not safe to follow. Said bush was not very dense, so we though multiple times: Ok, this could be the way. Or that path?
I believe regulations require that everything you put in or on food is technically editable. Like the paper stickers on bread or produce. They are disgusting, but if you or your child accidentally eats them, they are fine.
Have you looked into how existing software handles captive portals. I believe, both Ubuntu (or Gnome or Network-Manager) and Firefox do check for such portals and detect real internet access. (They simple poll some URL http://detectportal.vendor.com and check for the expected return code. Portals usually redirect.)
Now I'm thinking, what if this check could trigger a change to the DNS configuration. That is use DoT when internet is available, otherwise fall back to DHCP announced DNS