There are far fewer tourists in Tokyo than just two months ago because of COVID.
--> this looks correct! you could also say, "due to COVID"
She has twice as many books as I do.
--> this looks correct! You could also say "she has twice as many books as me"
I became more scared as the doctor explained (about) the surgery in more detail.
--> I would remove "about"
--> I became even more scared as the doctor explain the surgery in more detail
My wight (weight) is half as heavy as his.
--> this can work, but depending on what you are trying to say, I would use "I am half as heavy as he is"