First thing that comes to mind is the SMD Challenge kit on Tindie: https://www.tindie.com/products/makersbox/smd-challenge/

That goes down to 0201, but starts at 1206 so you can get warmed up. Little pricey, but they do have volume discounts.

Good luck, interested to see your results.

Well it doesn't have to be a design that's already done. Designing a custom PCB for this project wouldn't be a problem. I'd then simply order a batch of 25 or so (that would already be enough for half a year)

True, like I said, just what popped into my head. The other thought I had was to pass the boards around to various people who can do the test at various times of the month. Could help get better statistics and possibly control for skill level.

Replying to your top level comment because I can't reply to the other one.

Passing the boards around might be fun, otoh if I just publish the design everyone can add their results to the pile.

I'm now leaning towards a PCB with no actual function that just has some 0201 footprints in two different styles (more and less prone to tombstoning) and with different thermal surroundings. I'd place a mix of caps and resistors, maybe 16 per PCB. That's plenty to get me dizzy for a week until the next is due.