Ever thought of trying to include Microsoft's new High Efficiency Image Format (HEIF)? Since Manifold is so Microsoft intergrated, perhaps it's something to think about.
Granted none of the GIS I work with has this feature, nor any of the current versions of the graphics programs I use have it.
I would also recommend using JPEG2000, as its an ISO standard and offers lossy or lossless compression with compression level support. JPEG2000 is not as fast as ECW in my experience but since it's an ISO standard it's likely to gain favor among some users. I still love the speed of ECW files, yes I'm an ER Mapper fan, have been for 17 years.
Since many in the forum tend to like the idea of open standards, perhaps JPEG2000 would be a good image support option unless you can get better lossless compression with faster display, but I am impressed with Manifold 9 and how fast it works... the internal storage format for imagery is far faster than that esri, but that's a low bar.