How often are you changing oil? How much are you burning between changes?
As you noted, Toyota low-tension rings sometimes get clogged with carbon and start burning oil. Additives and flushes can do a decent job of freeing up sticking rings, but if the oil return holes in the piston get clogged they pretty much have to be cleaned out mechanically (brush/pin vs chemical solvents).
Here is the piston from a 270K mile Highlander hybrid that still had pretty good cylinder crosshatching but showed signs of burning oil. This was taken after the piston & cylinder were soaked in WD40 overnight:
This next photo was taken after the piston was soaked in a jar of Kerosene overnight and then in jar of Simple Green the night after that. I also spent a few minutes brushing the ring lands with an old toothbrush before starting to use the old ring to scrape the carbon:
^Note that the oil return hole is still plugged even after I used a ring to scrape the carbon from that section of the ring land. Ultimately I had to use a needle to poke a hole through the carbon and then use a tiny pipe cleaner to brush away the remnants. Here is a picture from the underside of the piston of the oil return hole as it neared the end of the cleaning:
That engine had 4 holes per piston, 3 of which were 100% blocked. The 4th was partially blocked, but appeared to respond favorably to flushes/solvent soaks so I think there is some hope if you act before the oil burning gets too bad.