I also think it's fine as is, the only thing that urks me about it is that the basic strategy described in the OP (one CH or HB in every city but not both leading to increasing trade route slots by one per city with no maximum cap) is one of the deal-breaking benefits in the wide-over-tall debate. The potency of trade routes (and inexpensive cost of traders) makes having more slots too valuable to not prioritize. Sure there are other ways to have more slots, a few civs like Persia and Cree get a single extra TR over the entire game, and very few civs (Mali, Carthage, and most prominently Portugal) get a few extra trade routes. There are also a few world wonders (Colossus, Zimbabwe, might be forgetting one) to add one or two more. And the other way of getting more trade routes is through grabbing the Great Merchants that add more slots, which as long as you can stay on top of your amenities make them the best GMs for this reason but alas, the accumulation of each type of Great Person points, including GMs, is another benefit of wide over tall.
Sure, you can play some civs like Kongo and Khmer taller over wider, but wider Kongo or Khmer are even better. The only one who's well-designed for tall over wide is Maya but my opinion only, I think the bonus and penalty numbers need to be swapped: +15% to all yields of cities within 6 tiles and -10% to all yields of further cities. I've tried them a few times, sometimes only settling cities that get the bonus and other times first prioritizing those cities and then having a few penalized "annexes" to give some supplementary trade routes, great person points, and gold and faith accumulation, and found that getting the +10% was noticeable but not impactful enough to make up for the handicap, and the -15% was enough to really question ever settling those cities.
Perhaps if there were some extra rules, like having fewer than X number of cities divided by Y world era, you could get extra trade routes from having both markets and lighthouses in the same cities (which would require an overhaul of the Owls Society) or that being below that threshold also made banks and stock exchanges give an extra trade route slot. Also, the audience chamber could use a rework - the ancestral hall is well-designed for wide empires and the warlord's throne is good for aggressive play, but it seems the audience chamber was designed for tall-over-wide play but just doesn't cut it. Also the diplomatic quarter and it's buildings could be reworked and with emphasis towards this end.
I don't intend to derail this thread from trade routes towards tall vs. wide, but the latter is derivative of the former.