My suggestion would be 1., but 2. is okay as well. The process to get a good protocol will be iterative.
I have a Spirulina culture arriving Monday, so I'll try some experiments locally, too. My process will be iterative: I'll set up an experiment with the best chance of success: decent initial density with a culture I know is healthy, warm environment (30C - 32C), moderate amount of light to avoid photo-inhibition or shading, 500RPM. Goal is get some (any) growth. If that works, I'll try with less initial density. If that works, then try a chemostat, etc.