We have a SO 39i so fairly similar with your SO40.
We upgraded to a 25kg Rocna. According to the specs a 20Kg Rocna would do it but we opted for one version bigger.
Regarding chain, if you now have 10mm, I would stick to this. This because your Anchor Windlass
won't take another size probably.
We have 70 meters G40 grade, which is 33% stronger than the standard G30 grade. If needed, I can add line when anchoring
in really deep water
and heavy winds.
G70 grade 8 mm will be sufficient form a strength point of view and will save you significant amount of weight at a place you don't want it. But it is costly, and as said, your gypsy/windlass probably needs to be adjusted.