I worry about terminology here. RDS and magnified scopes should both have the graticule (a reticle is a rangefinding graticule) in the same focal plane. If focusing on the dot or crosshairs (regardless of actual shape) makes the target at all blurry, or vice versa, then something isn't set up right.
RDSs are supposed to be all set up properly from the factory, so no adjustments, but your individual eyes may vary. I can no longer use Aimpoint, as I just see a starburst. If you can't use an RDS well, try another brand.
Magnified optics have to be focused to your eye (occular focus or diopter adjustment), and may have to be adjusted to focus at the target distance with a parallax adjustment. If you find the target, the graticule, or both to be even slightly out of focus then you need to change adjustments and fix it. Note, your eye will try to compensate so if you ever find them out of focus then you need to fix it; if tired, dehydrated, or after a long time looking through the scope it's hard to stay in focus, that means your adjustments are a bit off, and your eye is getting strained from trying to compensate.
Okay, once set up, then the activity your eye/brain is doing is rather different that focus anyway. Instead you should just be aligning the graticule with the intended aiming point.
There should not be a need to shift between one or the other. People do very well aligning objects, and do so dynamically, with not only no slowdown from shifting attention, but actual increased speed from having aligning items; think about people who read by tracing a finger, pencil, or ruler to a printed piece of paper.
Centering alignment we do very, very well indeed. For the question about doughnut graticules, you can indeed treat it very much like using a diopter/peep/ghostring sight: your eye will automatically center the aiming point in the ring, so just look through it and fire.