Spherical is not far away and you need some work to smooth it out anyway so it is not really any extra work to get to a sphere. When surface is smooth and edge good go slowly toward the parabola. The parabola is very close to the sphere and a parabola can be reached by with very little work.
Zones are sometimes caused by poorly pressed lap or closing channels. If you have those things right the next major cause for zones is using the same stroke pattern and size for too long a time. Too long time is relative. Now that polish is complete, after one or two turns of the mirror when on top or around the mirror when TOT is usually about time to change the stroke in some manner. Frequent change is good so I usually choose to change after each full turn of or around the mirror. For figuring I use almost exclusively W strokes of various stroke lengths and widths of the W. The Ws usually will be either 5 or 7 back and forth strokes across the W. Whatever size W and number of strokes I've chosen I do it the same at about 12 (between 11 and 13) increments to a full rotation of the mirror or trip around the mirror. Sometimes (but rarely) I will use a center over center stroke, usually either 5 or 7 strokes at about 12 positions, but sometimes for any straight back and forth stroke I will do a single back and forth stroke at each of about 60 increments of mirror rotation MOT or positions around the mirror TOT.
For a 6" mirror typical stroke rate will be about 1 forward and back stroke in 1.5 seconds, so about 40 strokes (forward and back) per minute. Shorter strokes like 1/3D might have a higher stroke rate, say around 1 per second or 60 per minute. A good rate depends on some variables and lower or higher rates than what I suggest can work ok, but you won't go wrong at the suggested rate.
For figuring steady pressure can be applied by hands on the back of the mirror as you stroke. Good pressure would be something between 3.5 and 7 pounds as you stroke. A kitchen scale or postal scale can be used to see what that pressure feels like. I put my hands palms and fingers down flat on the back of the mirror with thumbs touching at mirror center. Thumbs are a good reference for path of the mirror's center as you stroke the desired pattern. For a 6" mirror the little fingers will have to float in space because there is not room for them on the back of the mirror. Very slightly damp palms will grip the back of the mirror well enough for stroking if pressure is applied evenly. Too much moisture, hands slip. I rarely ever hook my fingers at the edge of the mirror. The same pressure is maintained during the entire stroke pattern and at each increment of rotation, unless I have some special reason to alter the pressure. Not likely to be much need for modulating pressure for a 6" f/8.
All that is the basics of what works for me.
Then there's choosing specific stroke length and width of the Ws
I'll say more about that later.
It should not be difficult to finish the mirror working only MOT. I recommend working only MOT until something comes up that is not working right with MOT.
For now if you work another session I recommend doing a few different size strokes, mostly small to work toward a sphere. The mild zones shown in post 1 images are a typical result (for me) when doing only COC.
So add some side motion to the COC to make them narrow Ws, width of the W 1/4 or 1/3D. Stroke length about 1/3 D and sometimes 1/2D long. But remember do the same length and width for full turns of or around the mirror. Change width and/or stroke length after each full turn.
It will take a while to remove the central depression in the center of the mirror, so I would say do the above for about 45 minutes and see how it looks after that.