If it's about making a civ weaker by having weak units in relation to the other civs over time, why not just contiune with giving all civs the techs (except special flavor techs for unique buldings or UUs), but instead of giving the barbs progressively better units, you could simply deny them the better units from the list, and then assign units that are weaker but they would still fit in the flow of time.
Good example: Rood & Dragon scenario -- Romano-british vs Anglo-saxon
The first set of units available to the RB kingdoms were superior than the set of units to AS kingdoms. But within the first age, the units for AS were getting better and better while the RB units weren't getting stronger (just simply replaced with units with the same stats). Then if I remember correctly, the 2nd age actually gave the) kingdoms units with worse stats. Predictably, the Romano-british kings waned in power for that scenario reflecting what had happened in history.
It's up to you. I believe your method will produce interesting info about the firaxis engine, but if you are still struggling without a solution in the end, this simple one will work for sure. I hope I'm not dissuading you from testing your theory. I'm rather curious what exact results would yield. The AI can be so bizzare, lol.