Fouled sparkplugs

I replaced my sparkplugs this afternoon as my 2011 328xi has 63,000 miles and has had a rough idle for the last month.

I was pretty surprised to find that cylinders 1 and 6 had fouled plugs (see pics). The others weren't fouled but had good carbon deposits and were degraded. By the look of them they should have been replaced at least 10,000 miles ago or earlier.

Weird things is that beyond the rough idle, the car was performing well. I'd think it would have been throwing codes and having performance degradation....

So, my observation is that the BMW recommendation of plug changes at 100k may be too long for many.

Also, what can I do differently to avoid this carbon buildup? (Note that I drive highway speeds everyday)

Also with that much buildup in plus 1 and 6 should I be concerned about those cylinders?

Thanks.

Fouled sparkplugs