Have been wearing the Apollo 8 on the strap it came with up until recently when I purchased the black canvas strap with deployant that comes on the dark side of the moon black black.

My issue is that I can’t get the watch and strap under my shirt cuff now with all the added bulk and I’m honestly not that big of a fan of the original strap it came with. The yellow stitching is a bit much and the buckle and holes are very widely spaced so it’s either super lose or way too tight.

Any good recommendations for a lower profile calf skin strap? Having trouble finding a 21mm option.

 

There are a number of good strap makers to choose from based upon your location and preference. I can personally vouch for www.thestrapsmith.com. I've purchased several straps for my Omega and Panerai watches and have been very happy with his quality. I'm sure there are a number of other makers different members use and are equally as happy.

As to the Apollo 8 specifically, I keep mine almost exclusively on this OEM NATO.

 

I would opt for the strap that comes on the new Speedmaster moonphase model. Matte black gator with white stitching.

I put one on my 9300 moonwatch (among the 6 or 7 other options I also picked up) and love it.