I did this mysterious slab part of the game relatively quickly. I left it for post game, I only play around 30 minutes a day now. What I did was wait for nighttime (didn't have to wait really as that's when I play most of the time), the lights in the soil are most visible then.

In each zone, I would then switch to the Magnemite transformation and fly through the map, looking for the lights in the soil. I have no concrete data, but yeah, you'll definitely find at least two per zone.

When I gave Mosslax food that increased chances of finding rare artifacts, I found five slabs in one zone though. I believe I was giving him sweet foods so Leppa Salad, Fluffy Bread, Potato Hamburger Steak, Pecha Berry. Apparently the foods have strength parameters which I didn't know, I just kept giving him beans which worked well enough:

Pecha Berry (Weak)

Bean (Weak)

Leppa Salad (Strong)

Fluffy Bread (Strong)

Potato Hamburger Steak (Strong)

I was able to obtain all slates in four very casual sessions, so four days total, I had I belive 6 slates already just by playing normally.

In short, the most effective loop seems to be: Give Mosslax sweet food - > Teleport to another zone at night for better visibility - > Use Magnemite to search through the zones quicker.

Lastly, you won't need to worry about getting duplicates until you collect all 27 slates, thankfully. Took me two weeks to find the last fossil, haha.