This is great observation and advice. I have a similar issue with my Malibu and Scion tC (with the notorious 2AZ-FE rings that get clogged). The malibu is GDI and super sooty. Either the rings are stuck or worn and it has blow by and burns a quart every 800 miles or so. Same for the tC, except it isn't GDI and those rings are known to clog.I took care of an oil burning fb25 6mt outback for a friend from 200k-280k the original engine consumed a quart every 500 miles until the last 10k it used a quart every 150 miles. The least consumption was with m1 0w40. No flushes or additives, cleanings, driving styles, pcv valve or happy dances changed the consumption rate.
When I pulled the engine at 280k the cylinders had 90% leakage at the rings from leakdown test. Found a low mileage used engine for him and it's done 30k since then with consumption less than a quart in 6k. Also with m1 0w40. My advice is to find the happy oil (which you may have done) and it will run much longer than one might think as long as they keep oil in it
I have tried all kinds of flushes and oil additives, including Auto-Rx, and nothing has really helped. Using a thicker oil MIGHT help slightly, but maybe not even. I have settled on 5w30 oil even though both engines call for 5w20 because I think its mostly calling for thinner oil due to CAFE. IMO, 5w30 is a jack of all trades oil thats great when cold and also when warm, without being too thick or too thin.
I will try one last thing in the Scion. I will try a piston ring soak with something like sea foam or gumout since I have to change the spark plugs anyway. I will remove them and put the seafoam in the chambers and let it sit and soak. We'll see what happens. I'll turn the engine over and do it again. If this doesn't help, I don't think anything will. The rings are either just stuck or clogged or worn and will stay that way until a rebuild, which I likely will never do since these have over 100K and run fine besides the burning oil.