First point will be you cannot assume ever that you will be allowed to enter Canada as a visitor for 6 months, even less so having already been in country for 3 months on a previous visit. Will you have a return ticket to support your visit.
Second even if you do stay 6 months leaving for a week visit to the US and returning it is highly likely that you may only be given a limited stay of a week of so or even refused entry given it will be pretty obvious to immigration that you are not a genuine visitor but trying maybe to live in Canada on a visitor visa.
Thirdly with such long stays in Canada as a visitor then any future application for a visa will take that into account when you apply and maybe rejected on the basis that you are not a genuine visitor spending such long periods in the country.
Obviously applying straight after your current visa expires might not be a good choice but even after a year assuming you are successful in spending so long as a visitor immigration will take a lot of convincing your application as a visitor is genuine.
I have no info on US immigration requirements but maybe check whether they insist on a minimum 6 months validity on your passport regardless if you say you will only stay a week or so.
US CBP as well may need some convincing about your intentions in entering the US regardless of having a US visa given any potential entry back into Canada issues that may exist but that is just a personal view.