Since rebuilding my S1 and making it nice I SORN over winter and it lives In a garage so its less exposed but galvanic corrosion is a major issue with the aluminium chassis mine had some serious issues, rusty fixings seem to have the greatest effect (probably because it holds moisture)
One thing I did change on the front undertray was to junk the spire fixings on the sides and replaced with a strip of stainless with rivnuts set in, saves a lot of fafing about with the front one as well as providing better support.
I use stainless fixings on my undertray with plastic washers under the SS washer very cheap and just replace them when they crack. I use either grease or a touch of wax oil to keep the moisture out. Don't use stainless for any load bearing bolts but there fine clamping any plastic bits.
The Aluminium chassis is anodised which does help reduce corrosion, but its easily damaged.
Note the rivnuts are aluminium, I've had to replace a few. My advice is use the serated ones and a little epoxy on the outside the slow/strong stuff this then bonds and seals the joint, never had one come loose again. Not sure graphite is a good idea, same for copper grease I never use either near the chassis just in case (don't know wether you can get galvanic corrosion from metal loaded greases as you need to have an electrolyte in contact, any tribologists out there?)
Best of luck, I remember also having to cut my front undertray off with a grinder and same with the wheel arch liners none of it was fun but well worth doing.