Update:
The app updated in June 2019 to support using a basemap sideloaded to the device during download. You can learn about this here: Prepare for offline data collection—Collector for ArcGIS | ArcGIS in the section "Reference an on-device basemap"
Original answer:
Hi Marcus - In 19.0.1 you can sideload a basemap and use it after the map is downloaded, but you can't use it as the basemap during download to avoid downloading the basemap the map was authored with (this is covered here). The ability to use it during download is coming to the new Collector in the release on iOS coming soon (this month).
As to your point on how you are required to download a basemap today, you don't have to download a huge one. Just download the basemap in the map with the lowest level of detail (like level 0 or 1) so that you get a smaller basemap during download, and replace the downloaded basemap with the sideloaded one when working with the map.
Esri Education Team