Hi guys ! I have a piece now and I am a bit afraid to attach it simply because my balding area is somewhere in the middle of the head, is not going to affect the part where the top meets the side hair...do you think I will have a problem with blending if I will cut from my piece the shape i've drawn in the image I attached? or it's better to shave more hair and use it as a regular topper?

Thats not dissimilar to my template. I had a scalp reduction and it narrowed the gap.
If anything, you should find it easier to blend if you keep as much hair of your own as possible.
Because its confined to the flatter part of your head, you shouldn't have problems with wrinkles and you won't get wear & tear from pillows & sleeping.
I take it you have an all lace base?

yes all lace base french withs swiss front

It'll be fine. Should be easy. Just cut carefully and take your time.