Going back to a START charge of powder and working up with a small magnum primer has been proven safe. If it were me, I would use a spherical (ball type) powder rather than a flake powder because sphericals are slightly harder to ignite, anyway. In SPEER load manuals you'll notice them recommending magnum primers in .357 Magnum even with faster burning spherical powders like W231 that produce considerably less velocity at near equal pressure to slower magnum type powders because they are slightly harder to ignite in the magnum case vs. use in something like .38 Special with lower case capacity and operating pressure.
I'm aware that you probably can't find any powder on the shelves these days, but two medium burning sphericals that I use and can highly recommend are Ramshot True Blue and Silhouette. They are both very pressure stable powders and won't give you pressure problems using the magnum primer. Especially when you are talking about loading the 9mm.