In Bulgaria over time are added new Avg speed sections, so not sure if you should manually do that or extract the sections from the description in BG Toll Сертифицирани отсечки за средна скорост | BG TOLL automatically but would be helpful if Waze actually calculate your Avg speed during the A → B section and the notifications before/during entering/exiting Avg. zone wazer to be notified regardless of if you set route or you use it just as map visualization
Thanks