It's absolutely fine to use Bilberry Wheel Cleaner on Santiagos or any polished diamond cut wheels. Used it for years on my Mk6 GTI diamond cuts, and any other wheels for that matter. It's a brilliant wheel cleaner and can be diluted as you wish. It's PH neutral, not acidic like some wheel cleaners. For regular maintenance washes I use it at a dilution of 1:10 which is effective in removing crap but not stripping protection. Use it at 1:3 for a more concentrated solution but that will strip wax and some other sealants off, except ceramic coatings.
A squirt on each wheel before your normal wash or at the snow foam stage (if applicable) will bring the dirt of with great ease. Also brillant on tailpipes.