Ready mix supplier here. Affirming your discussion, that you can certainly go higher than 30% SCM for concrete polished slabs.
The 2 biggest factors are (1) time to initial set and (2) time to final set.
SCM content is one of the factors, but not the only factor. For instance source of SCM is a big deal, fly ash on the East Coast is different than the West Coast. West Coast slag can replace cement at ~1:1 weight ratio. More East Coast slag is needed to replace an equivalent weight of cement. Temperature is also a big deal.
All that being said a lot of communication is critical if you are trying to reduce the carbon impact of your concrete. Certainly a lot of finishers feel comfortable <30% SCM, because they’ve always done it that way and it would be on them if the work wasn’t performed properly. However, that will artificially limit your ability to make your projects more environmentally friendly.
Ernest Maier and Bay Ready Mix