This probably calls for a longer post, but here's the short version.
- QGA default base map is in web mercator EPSG 3857
- Default feature layers should be in WGS84 EPSG 4326
- QGA can handle background and feature layers in other projections using base map "Empty" meter or degrees
- If a custom projection, then all layers should follow the same projection.
- No reprojection takes place on the client.
So for instance, using your CRS EPSG:28992, it's a meter based CRS:
- Switch to base map Empty meters
- Load a background WMS in CRS EPSG:28992
- Load coordinates from an excel file with EPSG:28992
- Connect to WFS, use EPSG:28992
Fyi, when importing data through the connector in other projections, the connector automatically converts to WGS 84, EPSG:4326.