I realize that you’re probably already committed to using 28mm figures, but I’ll throw this suggestion out for consideration- what about using groups of 10mm or 6mm figures on each base, then playing the rules as written?
Two or 3 strips of 6mm figures on a 20mm square base will give a suitable mass battle effect - around 120 to 180 individuals per 12 base unit. There’s nothing in the rules to stop you placing the bases side by side and a 6-wide, 2 deep formation of sparabara (or Macedonians), would look quite imposing. Fewer figures per base would be needed with 10mm models, but the visual effect would still be impressive.
I actually started a project along those lines using 6mm figures (different rule set) and liked the look. I got distracted before it was table-ready, though, so it’s all theoretical as far as how it would play.