How to thinly slice chicken thighs?

You are certainly correct to cut against the grain.

Certainly slicing meat would be difficult with a dull knife, but I have no way of knowing how sharp your knife is from reading your question. While "sharpening" a knife with a ceramic mug may be a neat trick in a pinch, you should look into a better solution. Using a whetstone is often recommended, or you could pay someone else to sharpen your knife occasionally. Having a reliably sharp knife is essential for precision and safety in the kitchen, and will make a lot of tasks easier.

Once you get your knife sorted, the usual trick to slicing meat thinly is to partially freeze it before cutting. These instructions from Japanese cooking website Just One Cookbook go into more detail about slicing meat thinly, but are primary aimed at large pieces of beef or pork. But the basic idea still works. Chicken thighs are on the thin side, so about 15-20 minutes before slicing, place them, flattened and wrapped, in the freezer. Once removed, they will be just firm enough to cut precisely. If they are too firm, let thaw briefly; too soft, freeze further.

Now, this is a matter of person preference, but for oyakodon specifically, I do not like the chicken cut too thinly. Paper-thin chicken overcooks easily, turning mealy and sometimes even breaking up in the broth. I cut my chicken to between 1/4 and 1/3 of an inch (0.6 to 0.8 cm) thick, cutting at a bias for more surface area. At this thickness, I can often get good slices without freezing.

There's a trick I use all the time for thinly slicing uncooked meats - though I'm definitely with Benjamin that having it just above freezing makes the task a lot easier. I have a meat drawer in my fridge which sits at 0.0 - 0.5°C & is perfect for this.

Use a fork.

A little similar to how you would cut meat on your plate. When you're first learning, stab your piece with a fork the width you need your slice to be & use it as a guide for your knife - literally drag your knife towards you, down & across the fork tines, just once per slice. Once you get the feel for the technique you no longer need to stab, you can just place your fork nearside of the meat & pull your knife through as though you'd stabbed it. It is remarkably precise, too.

I started doing this 20 years ago because I don't like touching raw meat, but have honed it to perfection since. I can finely slice something like chicken breast faster than I can using the traditional method of using my fingers as the guide, & no fear of nicking myself.

I always use a boning knife for this - because it is skinny & has minimal drag, so it's less likely to pull the meat out of shape as you cut. Sharp enough that each cut is one pull across the fork, no sawing.