So yesterday I decided not to use the monofiliment thread because when I did a test patch, I was able to pull it apart with some regular force (couldn't do that with the same piece and cotton or poly thread). The mono thread is very fine. I have 4 colors on hand of regular thread, gray, black, white, and turquoise. Since the back was predominantly black, I went with that. I did SITD. As you can see by the front picture, I, 99% of the time, stayed in the ditch (yay!!!) so the black never showed. The only stitching so far on the front is the long seams and not the short ones.
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But since some of the back is in color and I had no mono, the black showed up. If it was just the cross stitches
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I could let this go. But since I was following the lines on the front and not the lines on the back. On the vertical lines, it showed up too much and obviously not straight.
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This does look horrible. Granted it is a practice quilt but I really do love how it came out. So here is my question. Ready for the overthinking?

Is it possible, and if it is, should I tack off the black lines where they end at the beginning of the turquoise and buy a thicker mono to replace that area for the back? Also, because my stitches are off (tension maybe?) I was planning on not quilting too close together so I would have a little puff to hide the stitches. The batting has a range of 1/2" - 3 1/2". The only way to do this I think, and this is where your help comes in also, is to tack down just the center of where the 4 pieces come together. That will give me 2"-3 1/2" tops of space between seams. I don't want to tie off like on some quilts with yarn. This is more modern/contempo and tying off is more traditional. My machine does not offer a decent decorative stitch to tack it down. Buttons IMO won't look good. So how can I do this? Any ideas?

I did run practice pieces for the tension but it seems the beginning was fine with the tension or at least it looked like it on the black but the more I sewed, the wonkier they looked. For this reason I am thinking when I buy another machine, auto tension might be important. This is also what is steering me away from the Brother 1500S or any mechanical machine where I have to play with the tension all the time.

Okay everyone.... set me straight.