If you don't like the look now, there's nothing wrong with taking out the stitching ou've done.
Better to take the time now, than to hate it forever!
How do I pick the thread?
I audition the thread, against both the back and front.
Pull some of the thread off the spool and just let it fall on the quilt top/backing somewhat like it would be when quilted. That is, about the same distance apart. Stand back a bit, and you'll get a good idea how it will look after quilting.
Contrast of thread is more noticeable, yet can be very effective.
A coarser thread is also more noticeable.
Both tending to show any quilting faux-pas!!
Wherease a finer thread is much less noticeable.
I've had good luck with the Wonderfil/DecoBob 100 wt and 80 wt threads when I don't want the thread to be a focus.