You bring up a good point about marketing.

Maybe version "names" could be helpful here.

Make a marketing "name" and use that as a way of showing off what's new, while still keeping semver true to it's name.

In your example, version 1.3 could be "Saucy Gorilla", while 1.8 could be "Delicate Apricot", and when you feel it's change enough from the start, you can pick an arbitrary point and make a new name and blog post.

It would still have some of the bikeshedding that older "major.minor" version schemes have, but it could be not as bad because it is more obviously arbitrary.