Rollen, first we need to know how many miles the car has on it and also how many miles you drive before you need to add a quart.
You said “At the end of a week and half I usually have to fill it up almost entirely”, but that doesn’t tell us whether you’re just adding a quart or four quarts. Nor do we know how many miles you drive in a week and a half.
If the car has 250,000 miles on it and you’re only adding a quart every 1,000 miles, you may not have a problem at all. But you need to tell us.
I can tell you that if you changed the exhaust system, including the cat converter, and the oil cap all in the quest to reduce oil usage, you’ve already spent a lot of money on things that do not cause oil usage. I recommend strongly that you do nothing else until you find out if the amount you’re using is normal (post the requested info) and/or get some ideas as to the cause of the oil usage (if it’s abnormal). Otherwise we’re all just wildly guessing and you’re just spending money for nothing.