One approach is just to open a service on my-domain.com on port 443, solely for the sake of hosting the /.well-known/assetlinks.json file.

It doesn't need to do any other stuffs, any other service can stay on your port-specific endpoints. It should work beacuse the deeplink urls will actually be handled by your app, and your app is free to point to any other endpoints (with the specific ports). The only time you need to go through the port 443 is when you need to retrieve the /.well-known/assetlinks.json file. This way, you don't even need subdomains.

If you have dedicated package name for every development environment, you can list all of them in single assetlinks.json