It took me another improvement of my quali time, about ten times of "one more race and that's it" and 4 second places, but I finally won in a daily race C this week.
My DR skyrocketed to 28k after that, so I tried a couple of races in higher-B lobbies and I think I achieved some decent results.
First race I started on P8 and finished on P8 as well, third race saw me climb up one position from P10 to P9. But the second race was definitely the best, as I reached as high as P3 after starting on P8, overtaking a number of other drivers, including the time I overtake two cars at once, which is a very rare occurrence on this track. Very happy with these results, as they generated as much as 29874 DR points in total for me, just a little bit short of A level.
I'm sure it will drop as soon as I switch the track, but I don't worry too much about it. As long as I'm in a group with drivers of approximately my skill range I'm happy, no matter the rating.
One thing I learned on this track is how to follow other cars within 0.1s and not touching them. That's the only way to wait for an overtaking opportunity, because every other move is impossible (at least as long as you want to be fair to others). The only decent place I found for overtakes is the penalty zone and turns before/after it. If someone goes wide going out of the tunnel, you can overtake him on this straight. Entering the tunnel after the penalty zone, some people go wide as well - and it is pretty possible to overtake them there.
Apart from that there are no overtaking opportunities, and the sooner you understand that, the better. I can't count how many times I saw someone on the radar trying to overtake me on the wide, only to hear them hitting a wall and seeing their blue icon disappearing. Each of this manoeuvres cost at least 0.2 or even more, which you'd rather save, so when an overtaking opportunity presents itself, you are there and not half a second behind. And yes, I did all the same mistakes in the beginning of the week as well, so I'm just sharing what I've learned

This logic is easily applied to other tracks as well, but Tokyo highlights it particularly well.
Another important thing is to know when you are beaten. If someone is side by side with you on the straight, and you are wide, just give way, because otherwise you are going to be smacked into the wall, no matter if other driver wants this or not. Avoiding this is very important, because being smacked into the wall slows you down much more than just a little tap on the brakes to let another person through. And as it is a very high-paced track, chances are that by the time you recover from a kiss with the wall, a person or two will overtake you as well.