Not sure if this applies to your situation, but I believe you can avoid a full rebuild by modularizing the flake.nix derivations into stages (calling a separate *.nix for each stage in my case). That is how it appears to be working for me on a project (I am building a cc toolchain without pkgscross).
I pass the output of each stage of a toolchain as a dependency to the next stage. By chaining the stages, changes made to a single stage only require a rebuild of each succeeding stage. The final stage is the default of the flake, so you can easy get the complete package.
In addition, I can debug along the toolchain by entering a single stage env with nix develop <stage>
Not sure if this is the most optimal way, but it appears to work in modularizing the rebuild.(using 23.11)
I pass the output of each stage of a toolchain as a dependency to the next stage. By chaining the stages, changes made to a single stage only require a rebuild of each succeeding stage. The final stage is the default of the flake, so you can easy get the complete package.
In addition, I can debug along the toolchain by entering a single stage env with nix develop <stage>
Not sure if this is the most optimal way, but it appears to work in modularizing the rebuild.(using 23.11)