Shamo, I'd try switching 'imperial_palace' and 'proconsuls_palace' for every instance in export_descr_buildings.txt ( and descr_strat if cities are large enough). Hard coded stuff generally relates to resource / building name not context so that should bring it in earlier.

Melea Lord, to make trigger activate later, you could either, make building queue longer by increasing building times and / or enforcing inter-dependencies so all lower buildings inc - all levels of stables etc have to be built first - if squalor becomes too much of a problem you can add health benefits to other buildings.
Alternatively reduce population growth in regions with relevant resource ( I assume that the relevant one is italy- but it could be rome or something else!?)
In normal modding I don't think there's anything to link this to a specific year, but you could look into scripting, as that seems to achieve more....