I would like to choose my own map for use with Survey123 instead of just the basemaps.
If I understand your requirement correctly, this has already been implemented:
Prepare basemaps for offline use—Survey123 for ArcGIS (Beta) | ArcGIS
Hi Ismael and Ruben,
We would like to be able to use other projections, RD_New (EPSG: 28992) in our case. We would like users in The Netherlands to take advantage of the Dutch high-detail basemaps, like the Topo RD, which is available online only. This is possible in any other app in the ArcGIS Platform I can think of and it would be really great to have this feature in Survey123 too.
Retrieving data ...