As the new skin dries up the tension is going up. So does the sound changes and gets more crispy and shrilling.
I even have to change the koma to the lower one to get more of low frequency response and reduce resonance. Sustain and sawari is good but the overall tone is a bit harsh. Is it normal?
It can be, but it varies.
Different genre of shamisen have different typical tightness. Different players have different preferences too. I personally enjoy a relatively loose skinning for the resonance, but tight skins have their place.
Additionally so-called “tournament” ready shamisen are extremely tight and crisp to the point of excess in my opinion. The tension is makes the skin nearly as hard as a stone.
I also figured out why the front skin should be tighter: not like Kyle explaining in one of his videos but for the following reason: the front skin resonance frequency goes down loaded with the mass and tension of the strings and koma. So in order to have max volume the frequencies of the loaded front skin and the back skin should match in order to resonate IMHO, so the higher tension on the front compensates the frequency change.