I think I remember some years ago I could define the bitrates of audio and more importantly video. With my updated version (18.04) I can't find these settings. There are two sliders with 'Quality' as label.
Please, where can I preset a quality in bitrate(s) that pops up as default for all my upcoming projects (for the next 6 months they will require a constant specific format).

For the rest, a good progress over the last years!

I don't know which Kdenlive version you use. In 18.12 and in 19.04 by enabling more option you can adjust the audio bitrate by enter a number (i.e 192=192kbit/s). The video bitrate you cannot direct enter. Either you create an own render profile and enter the bitrate direct in the profile or you check the rendered file with i.e. MediaInfo to see the exact bitrate and adjust as you like. In 19.04 quality 23 is a good staring point.