Thread weight makes a difference in how clear the lettering reads. The columns are identical except for the thread weight. Top letter A is 10mm, 7mm, 5.5mm. 80wt thread is from Wonderfil and is lovely in top and bobbin.
If you have a thread trimmer on you machine for jumps, turn it off if you care what the backside looks like. No one is paying attention to those threads dotting the “i” or other in word threads. Use fray check on back side and carefully trim between words.
100wt thread in bobbin can make things even clearer. Yes you may need to adjust bobbin tension if your machine has a tendency to put bobbin thread to top. It’s not hard – read your manual and make micro adjustments.
I use a SCHMETZ gold 75/11 embroidery needle 95% of the time. I may switch to a Microtex needle if stitching on a tight woven but that’s rare.
Starch. Create a stable surface to embroider on that’s the key to successful embroidery for any project.
Be smart. 10mm lettering on a fleece is not going to show up – make a patch and appliqué 🙂. I’m not being mean when I say be smart! We all get too close to project and don’t see obvious. Breathe. Take step back and look at the entire project and say what is the best way to approach? If you’ve tried thinner thread and needle and it still is not right, maybe the material stitched upon needs to be adjusted.