Who actually sides with Meredith?

May be the problem is you don't need something on a macro-level. If each nation was able to decide for itself, it is likely each would come up with their own way of dealing with the problem. They have already done so in Rivain, where the old Circle system and the Chantry was only really in control in the capital. You also have Nevarra where the Mortalitassi hold a great deal of power but are apparently (according to Tevinter Nights) happy with a situation where non-mages have the tiresome job of running the country. I suspect they were established before the Circles became closed prisons in the south, which is why it was not possible to remove them thereafter.

If you study the history of the southern Chantry in WoT it is clear that the original Circles were very much run on Tevinter lines. They were somewhere that young mages could be educated before returning to the community, plus somewhere more advance magical experimentation could be carried out under Templar supervision, which was a sensible way to ensure tragedies didn't occur if it ran out of control. (Something Justinia ignored when she authorised Pharamond's experiment concerning tranquility with horrific results). Drakon permitted mages to live in the community and even serve the ruling classes, so long as they did not unduly influence policy, did not become rulers themselves and did not practice blood magic.

It is pretty clear they only tightened the restrictions on mages in response to the rift with the Chantry in Tevinter. However, just because of what occurred there, did not necessarily translate into mages everywhere wanting to take power. One of the problems with the Circle system is that it makes mages regard themselves as a group separate from their communities rather than part of them. If you let mages stay within their communities, where they would definitely be in the minority, they are more likely to identify with the other people in their families or village groups rather than automatically teaming up with other mages. Most mages have only limited magical power so would have no reason to be overly ambitious and quite content to confine themselves to healing and other beneficial magic.