Selam aleykum
Akhi, sehri is not a time. One can eat until the white thread is distinguishable from the black one. That is the beginning of Fadjr. So that also means you can brush your tooth up untill fajr, and not only before sehri. Doesn't make much sense either to first brush and then eat.
As for brushing teeth after Fadjr, I don't know, there seem to be different opinions regarding it, I don't know which one is right so for now I'm avoiding it. And allah subhana wa ta'ala knows best.
