Sound fonts are quite old and fall behind on quality these days; in addition Akai samples are getting more and more difficult to come by as they have been pretty much replaced with sample VSTs. (Kontakt being the most popular)

If you want great sound expansion capabilities then to be quite honest the only way to go is to have a VST Host built into the keyboard so that you can have what you want. (I wouldn’t consider any arranger/organ without this basic option built in)

Bill