Does 10 plus have touchscreen issues on salfish after a10 install? And why need to install a9?
Welcome to the forum. With regards to Android 9 installation you might like to look at this resource concerning baseband:https://jolla.zendesk.com/hc/en-us/articles/115004283713#Baseband
why android 9 is better for instalation then android 10?
it is not better, it is mandatory.
sf is built for android 9 firmware and drivers.
The touchscreen issue you probably read about is hardware related.
So, no worries when installing SFOS.
one of those threads https://together.jolla.com/question/225481/xperia-10-plus-ds-random-freezes/
After insalling a new battery my touchscreen problems went away.
I think it’s a question of bad assambly by the manufacturer in the first place, possibly the battery-plug cover.