Thank you for your insight.
In reality I used to use one (or sometimes two) back and forth city bus lines. I recently tried the circular ones, but I’m unsure which one is better.
Back and forth lines have the advantage of requiring less lines (one line cover both directions, circular lines require at least two lines per city, but circular lines may allow for shorter trips between zones as one can always take the direction that optimizes travel time (in your image from station to north part of city one must do all the trip, with circular lines it could go quite directly), but I am not really sure as back and forth lines can run more straight and you can probably have higher frequencies with the same number of busses.
An interesting thing is that when made back&forth lines I made them a little differently than you in that I often started from one district (e.g industry) pass through residential/station and end in another district (e.g. leisure). Since citizens always go back and forth from residential district to the other ones (that’s what the developers told if I understood it correctly, please correct me if it is untrue), in that way trips from residential to other district are shorter.
I also make much less stops than you, especially late in the game. I often have 6-10 stops per line (that makes 12-20 stops per city). I am not sure what is better, but with more stop trips are much longer as busses must continuously stop and restart. This also raises frequency. On the other hand you reduce walking time, so it is hard to find a balance.
I have not really tried to dedicate lines to each district. This could lead to much better coverage (other players claim to easily achiev usage rate of 90% or more, while I usually reach 70-80%) but it also may need more busses and this maintenance costs.
By the way, you should definitively try hard difficulty (if you like the business aspect more than building a nice looking transport network). The other modes simply don’t give any interesting challenges. Even with hard it is not that difficult to make some profit, but you have to manage it with some efficiency.