What is the difference between EQMod Align and Alignment Subsystem options? How do they each relate to the Astrometry alignment solver? And is there an advantage to using one over the other? The
Alignment with EQMod & Ekos
page doesn't refer to this option.
Before INDI Alignment Subsystem was developed, EQMod used its own alignment math. Now you can select between the EQMod one and INDI's official one. I didn't test the INDI Alignment subsystem extensively yet. To be "safe" for now, stick to EQMod align.