How about inverting the way National Wonders are usually done? This would reflect that with finite resources, certain things are possible only after a big project is committed too and finished.
Let's say the first economic civic you can have up from 'Subsistence Scaveaging" would be Barter. After 'Currency' is recreated, the National Wonder (NW) "The Mint" is creatable. After the creation of that, you can then build banks. I mean, if you don't have a place to make money, why build banks? You can't just say, "Well they'd just use old money" Backed up by what? Having "the Mint" shows that the building Civ has gone through the process to figure out what to tie what to their currency (like water in the movie "Tank Girl") to make people accept it.
So maybe after the slavery civic is created, you build a NW on the order of a Roman Slave Forum (sorry can't seem to find a name for a historical big slave center) and that enables you to build local slave pits/pen and perhaps these pits can allow the creation of slavers.
I think by using NW in reverse, would slow the 'release' of different buildings and those units associated with those buildings. I know that technically the research that goes into a tech is supposed to represent some of the money/effort to create the infra-structure, but knowing how to do something and being ABLE to do it is another. As an ex-military intel type, let me say it's 'easy' to make mustard gas; it's VERY hard to weaponize it. So maybe the NW "NBC lab" is needed before any sort of chemical weapons can be made since the NW represents the one place that creates the specialty tools or specialist needed to be dispersed around the new Civ.