Reducing it to one TR per city would solve a lot of problems, even without other tweaks. It would still be the case that the first thing you'd want to do when building a new city is get a trade route going. I actually have no problem with that -- it makes sense to me. And I like quickly growing cities. The system breaks with 2 and 3 trade routes, though, as you get both a quickly growing city AND an external trade route or two, for + gold and + science. It means that every single new city can both support itself and get +10, +15 science, which is huge, and makes you a chump for not ICS'ing.