Scarlet and Ed's really cool comics topic

I don't think you're missing anything too important by not fully reading the tie-ins. The only thing is that the main Metal issues tend to carry-over from whatever the previous tie-in was so it might take a second to figure out what's going on.

The Batman one-shots were pretty self-contained to start, but the last few had a lot more current universe stuff going on. The Batman Who Laughs is completely skippable in terms of overall importance, but I do think it's the best of the bunch.