To use all 24 channels you need to add 16 additional DA converters (digi --> analog), so the i/o options are using the two ADAT outs on the FF800 = 8x2 channels.  Something like an RME M-16 DA or Apogee DA16x would do this I think?   If your mixer doesn't have D-SUB inputs then it's buying two D-SUB --> TRS cable snakes as well.  That would get you total 24 analog outs for summing OTB.  Probably around $4k US or $3-4k when sourced used.
I currently sum via my FF800's 8 analog outs ----> Dangerous Music D-BOX, and it works pretty well.  Never thought about doing 24, don't do enough large complex mixes for that.  If you upgrade come back and tell us how it goes.  Some people will tell you to get a Big Ben wordclock or other stuff, but I think the FF800 is probably more than up to the task.