I do not have a clear understanding of what causes resin prints to become brittle. Firstly, it appears excessive cold (in the 40s or even 30s, I am in New England) may be a factor. What else can cause brittleness in resin prints? Is there a difference between resin types?
Many printing resins are inherently brittle.
Brittle is a property of the material - so of the used printing resin.
Look at the first seconds of this video - Punished Props Academy drops an SLA/DLP printed dagger on the ground from about 1 meter and it shatters, despite having been carefully post cured and washed and everything. To mitigate that trouble they did a cast of the item with a different resin.
However, the exact brittle behavior of the print is inherently linked to the exact formulation and thus the exact brand and type of resin used.
Some are less brittle than others - and outright flexible.
You might need to test around, but there are flexible resins. This german essay contains pictures and even a video of a super flexible printing resin. Just an example: One such rubbery resin is Formlabs Flexible 80A, and Matterhackers has a range of different flexible resins. Google does show other brands too by googling "SLA Resin Flexible"
I am not affiliated with Formlabs, Matterhackers, or Techstage.de