There are limits to the garrison benefit.
True. Especially when you hit the 12k or 24k mark. The method of adding one as needed is probably better <12k.

I've never liked the idea of vacating a city, letting it go rebel, then besieging it anew and exterminating the population. Some RTW players like that method of city management, but I prefer other alternatives.
I used to do this with any faction I played to the extent of having a roving "Exterminator"...a general & army who spent his entire career slaughtering city populations. Oh the monikers those generals would get "The Butcher", "The Murderer", etc.

Now I only do it when there's no alternative (Scythia gets no good law temple, for example, and Tanais & Chersonesos require periodic slaughtering due to the grain bonus)

I think this is built into the game design based on the city's historical reputation.
I believe it's 30% for this @#$% place! But I can get ZPG even here, so I rarely have to exterminate. Patavium is another like Jerusalem, IIRC, though for some reason I've never taken the time to see what the built-in unrest # is