One important thing to have in order to make a developer's life easier (both app developers and ubports contributors) is an easy to use SDK. I think we'll later discuss about the various options, but one common trait which is needed to get good performance and reliability is being based on a single image (as opposed to retrieving thousands of debian packages via apt).
The SDK team at Canonical was preparing such images, and putting them online at https://sdk-images.canonical.com/, where the client SDK could retrieve them from. I would suggest that we continue doing the same (and possibly fix the bugs that occasionally are found on such images!) on our own servers, as I suspect that Canonical might shutdown that subdomain at any time. At the address above you can find the script which was used to generate the images; I already downloaded a copy of it, but maybe we should put it somewhere in github.