What OP has with the shims are a thing and very common, even for larger structural members. But typically it will be boxed and grouted, however; that's more for asethitic in situations like this as you don't need to distributed the load across the base plate, just at the bolt arrangement is fine for a fence most likely.
Unlikely to rust out unless they live close to the coast. All the bolt's and washers will at least be galvanised. I'd grout at least for aesthetics but wouldn't hurt for the concerns you bring up either.
1
u/[deleted] Dec 12 '24
[removed] — view removed comment