The main problem with this is that your main source of starbases is the number of systems you have. Which means this will only weaken small empires (who generally have fewer starbase capacity) and relatively benefit large empires, which doesn't deal with the original problem of snowballing, but rather actually causes more snowballing.
Not necessarily, because you have ascension perks such as +5 starbases which greatly change in value. You also have traditions like expansion, which could have a competing branch so that expansion is going wide while something else is going tall. You can also always adjust the formulas for starbases from systems versus more starbase from techs. That tuning would all be relatively easy to implement, and probably balance.
The biggest factor is that if tall is to be a thing, you need a way to have a high concentration of large planets in nearby systems, as well as ways to scale bonuses on resources extracted from them to incentivize staying population dense.