Using -Dbuiltin_openssl=On brought up the same error, but -Dxrootd=Off fixed the error. During the build I did get an error with clad, but retrying with -Dclad=OFF fixed this error and I think I am ready to use SOFIE!
Many thanks for your help Lorenzo!
To summarise my SOFIE installation process for anyone else struggling:
brew update
brew upgrade
brew install protobuf
brew upgrade protobuf
protoc --version # my version was libprotoc 3.19.4
git clone https://github.com/root-project/root.git root_src
mkdir root_build root_install && cd root_build
cmake -Dtmva-sofie=ON -Dxrootd=Off -Dclad=OFF -DCMAKE_INSTALL_PREFIX=../root_install ../root_src
cmake --build . -- install -j3 # I'm using a dual-core device, so using j3. Use >=j4 for quad-core or greater
source ../root_install/bin/thisroot.sh
root