I can see how that could work.
On a related note, apparently the idea of slavery being immoral was completely unheard of in the ancient world, even among people who had been slaves themselves, so you could make the Separatists the ones against slavery. Might not work as well though.
What does the real life ancient world have to do with anything
The Republic is fairly analogous to ancient Rome.
In some ways yes, but then what are the Separatists analogous to?
The Republic is much more of a “modern” government and gives a lot more sovereignty and representation to each of the provinces/countries/planets than Rome would.
Most importantly, the Republic is supposed to be mostly sympathetic to the audience. Otherwise we don’t care at all when it gets destroyed and we don’t really want the Rebels to bring it back.