6/9 casters with +3 to spell DCs - fixed, or broken the other way?

[Pex]

Partial classes get (a) a BIG action economy advantage, (b) more class features, and (c) better defenses to compensate for the fact that they get lower-level spells and lower save DCs. This is known as a trade-off and it increases diversity between classes.

There is no problem with the 6/9 casters and nothing that requires fixing.

Exactly. In addition, while they might have attack spells that is not their focus. Their class features define what they're supposed to be doing. War Priest wants to be a warrior. Summoner wants to bring warriors to the battlefield. P1 Alchemist wants to throw bombs or be a melee warrior and possibly both. Bard wants to buff others. A War Priest will never cast Miracle. A Summoner will never cast Wish. The players of such classes Don't Care.

The only problem with hybrid casters is that nobody ever made a hybrid blaster.

The Arcanist can be, but it may depend on what qualifies as a blaster to you. Nevertheless they can choose class abilities to provide lots of blasting opportunities. However Arcanist is a full 9 caster, so while it is a hyrbid it's not a 6/9 for the thread.