I maintain about 20 industrial machines at work, but almost all are heavier-duty walking foots, and we use heavier thread, so I'm no expert on lightweight stuff... Or darning, but that shouldn't matter.

Does your repair guy specialize in industrial machines? Or is he typically working on home machines? "Eventually my timing was off" is a little concerning, the timing on an industrial should last for a long time if it's set right and nothing catastrophic happens (it's all metal parts, and if the screws are tight, nothing should move).

I have suggestions about what to adjust on the machine, but that's more for the mechanic, unless you want to work on it yourself. I would increase the needle drop timing a bit more than the manual says (by probably 1mm, or play around with it), max out the check spring motion, and lower the needle bar just a little bit (again, 1mm) so the thread loop is lower for the hook to grab. My reasoning is that your "darning" is changing the direction the needle thread is being pulled, which might be affecting where the hook is meeting the thread loop. My guess is that the hook is so close to the eye, the tip is nicking the thread occasionally, or just one strand, and breaking it completely a couple stitches later. Your photos seems to support that.

It also looks like you are using a really long stitch length. That too is reason to account for more "slop" in the thread motion.

Other things to check: Appropriate needle size, smooth thread path off the cone.

Sorry if that's too technical! It's just what I would look at, based on my experience. Industrial machines are endlessly adjustable based on what you're sewing, so there is no "right" setting for every project. The DDL-8700 should be able to do what you're trying to do, though.

If you want to work on it yourself, I could help with that, too. Everything I mentioned can be done with a single screwdriver.