I am just back from what was meant to be a long snow shoe trip, but it got aborted. A story for another day.
Now, your sewing. As far as I can see from your pics, the machine is set up correctly, the needle is in correctly, the needle size is fine (I use #60 on silnylon and #70 on slightly heavier nylons), and the thread is OK. I prefer a lighter thread than Gutermann for silnylon, but I do use Gutermann for Taslan fabric for clothing. And your description of the bobbin tension is good.
Now, what is happening? The sewing machine is running fine and doing its thing. I think the bottom tension looks OK (for now). But the top thread is lacking in tension. It is not being pulled very hard at all when the needle is retracted, and that is why the stitches look the way they do.
OK, if so, why? The same thing has happened to me a number of times. In each case I was able to fix the problem immediately by cleaning out the upper tension system. I slackened off the tension dial and ran some fabric strips and/or heavy thread through the tensioner thread path to clean out accumulated fluff and muck. It seems that stuff can accumulate in there and prevent the tensioner from putting the right drag on the thread. Also I took the opportunity to take the top plate off the machine so I could oil the moving shafts inside, including the little thrust shaft in the middle of the tensioner itself.
In an extreme case you might have to disassemble the tensioner to clean it, but this is to be avoided if possible as re-assembly is sometimes a bit tricky. Been there, done that...
You can tweak the bottom tension to compensate for manufacturing differences and huge variations in thread tension, but in general this is not necessary. You might want to tweak it a bit until you have a satisfactory setting, but after that it should not need touching.
However, I do tweak the upper tension a fair bit, to compensate for the different fabrics I am sewing. I prefer a quite light tension for most UL fabrics, to avoid puckering the fabric. Puckered seams are a failure point imho.
Hope this helps. If (and when) you get it sorted out, let us all know!