Hi Frank,
What a coincidence! I am working right now on a new astrometry-based mosaic construction tool. This tool will be ideal to generate extreme wide-field mosaics, such as the one you have captured. The principle of operation is similar to the MosaicByCoordinates script, written by Andrés del Pozo, which you already can use in PixInsight. However, being written in C++ and fully multithreaded, the new tool will be much faster. It will include also an efficient frame blending feature based on multiband (multiscale) blending.
So I ask you for patience while I work on this new tool. In the meantime, you can of course try with the MosaicByCoordinates script, which should also work nicely to generate this mosaic.