Automated solutions to this are in the works. What you can do in the interim is have brms give you the Stan code and data json files for the model you specify, then use cmdstanr to start the chains while telling it to write the chains’ output to csv (it actually always does this, but by giving it an explicit location, they get saved in a non-ephemeral location). Then if the sampling ends unexpectedly for any reason, you can use/extend the code here to resume (though you may need to work out how to parse an incomplete csv; I’m not sure if cmdstanr can do that automatically).