Id like to propose NON-full casters to be allowed to add their spellbook to the Mythic Paths just like full casters are able to.
In my opinion, since the system currently disallows to do this, it makes Classes that have a non-full casting progress less appealing.
Of course there is the Abundant Casting Mythic Feat and with 2 spellbooks you get 2x as many additional spell slots however I think that Mythic Feat / Ability is too powerful to begin with and might end up being nerfed. Either by only adding the slots to 1 spellbook (selectable) or adding less slots.
I think if the option were added to combine spellbooks instead of taking a Mythic spellbook standalone, it would greatly benefit the Classes without 9th level spells.
It would also make some Mythic Paths more accessible whichs Mythic spellbook uses a caster stat that is very low due to the Class having a different stat. If these Classes could combine their spellbook as an option, they could use their classes casting stat and have a better access to spells.
Below is a table I have modified with extra rows and columns. Please note that level means character level + Mythic Tier. The 10th level spell slots are MY GUESS. Left is full caster, right is 6th level caster.
As you can see, Ive added 7th and 8th level spells to the non-full caster (right table). Since there would still be NO access to 9th level spells, a full caster would still have more spells over this when selecting the Mythic spellbook as standalone.
Here is an empty version.