Hi @aschuma, welcome to the community and I am sorry to hear you’re having trouble here.
The documentation for VUE_APP_URL_BASE_API suggests this variable is only taken into account when building n8n:
Used when building the n8n-editor-ui package manually to set how the frontend can reach the backend API.
So you might need to build a custom version of n8n as described here. Tbh, this seems like like a lot of effort, especially when keeping these up to date.
So it seems much simpler to set up individual subdomains for each of your n8n instances (your third option) and just use the default path / for each (at least with nginx or caddy as the reverse proxy). Seeing you are using Windows Server make sure to check out this configuration advice from fellow community member @lublak if you want to use IIS: