Looking at the galvanic chart is only part of the answer. SS forms a passive film that is less conducting. Bronze does not. Since electrical
contact is nessessary the film on SS reduces the galvanic action compared to what would be expected from the galvanic tale alone.
So in general SS is slightly better than bronze. The overall design however is poor. SS can produce acceptable results if the aluminium part is massive compared to the SS, say in case of a mast
with a SS bolt, but that is not the case here.
Use lots of Duralac, or Tef-Gel (depending on your religion) and if you can minimise the contact with plastic bushings etc. A thin plastic washer und the SS one helps. Keep the area as dry as possible it needs to be wet for a problem to occur.
BTW Stumble titanium also forms a very good less conducting coating so Titanium produces less galvanic corrosion
in adjacent materials than its placement on the galvanic table would indicate. It's an important selling point as many people are horrified at using titanium fasinings because of its placement on the galvanic table. Titanium can still produce a severe galvanic reaction because the coating is less, not non, conducting.
If you could devise a totally insulating coating for titanium say a sort of Inert anodising we would have the perfect marine
fastening. All I ask is 0.01c from the sale
of every bolt and shakle with the "Noelex" coating to keep me in my crusing lifestyle.