I think the magical function you’re looking for here is Union(). It’s built into Excel VBA, so look at the help for it. It does just what you’d expect.
Loop through your ranges, but instead of building a string, build up a multi-area Range. Then you can select or set properties on the whole thing at once.
I don’t know what (if any) the limit on the number of areas you can build up in a single Range is, but it’s bigger than 600. I don’t know what (if any) limits there are on selecting or setting properties of a multi-area Range either, but it’s probably worth a try.