Brittleness has been my issue with mixes of rosin and beeswax alone. Instead of my thread becoming momentarily slick as I pull it through an insole, I get little puffs of dust and small crunchy flakes coming off.
All of this seems to really depend on local conditions. The old sources talk about seasonal adjustments to recipes. I expect both temperature and humidity affect how mixes behave.
I think one thing that helps with this recipe is that I’m not using commercially refined rosin. This is literally pine resin from trees, melted at low temperature and filtered, then allowed to cool. it still contains some residual terpentine. I can break it, so it is somewhat brittle, but I can also bend the shards, and they sort of fuse together if you leave a pile of them overnight.
At this point, several days later, in the cool morning temperatures, it is very firm stuff, but I can press my fingers into it if I squeeze hard enough.
But that may change over time, as the terpentine evaporates. I may need to re-melt and add a bit of fat or a few drops of terpentine eventually.