Your drain pH combined with aging time has more to do with postacidification fault than terminal pH pre brine. That said, for a tomme style with high drain pH (6.3), it should naturally finish around 5.2-5.4 within 12 hours, after which it is best to brine. Not the end of the world if you don't, but with raw milk, it makes for a more consistent product to drop the pH quickly and salt quickly.
Clostridium will find other food if there's no available sugar. Will still blow. More of a pH issue and flora balance for the spores.