As a final thought.
It seems problematic for Playtest 7 to introduce the concept of spellbook unreadability. I suppose it is a throwback to the Read Magic spell of earlier editions. 2014 seems to be better off discontinuing the concept. It seems awkward for 2024 to dredge it back up again.
A better, "modern" approach, does better to utilize the 5e skill system.
For example.
Each Wizard learns to do magic following local cultural traditions and personal tastes. In other words, magic works similarly to artwork. There are artistic styles, genres, and histories. But ultimately, each artist is unique.
When Wizards look at someone elses spellbook, it is normally fairly obvious what each spell is and what it does. So something like an Intelligence (Arcana) check for DC 10, at least for the low slot spells, such as a slot 0 cantrip, to recognize a spell. Maybe DC 11 for a slot 1 spell.
But to figure out how to reproduce a spell − to actually do it − is more difficult. Then an other casters spellbook is more like clues for how to do something. Possibly the ability to "know" the spell (putting it in ones own spellbook) is base DC 15, or maybe even DC 20. With failure representing the loss of time to research the spell.
But the spells that a Wizard gains while leveling, are personal spells that the Wizard was working on already, and these are automatically "known".
An example of the process, the spell from Shakespeare: "Bubble, bubble, toil and trouble." It is fairly obvious what the purpose of the spell is, some kind of curse spell, DC 10. This is being performed by a "ritual", instead of being cast with a spell slot. But the context of the spell might give enough clues for a particular Wizard to figure out how to cast it oneself. When successfully reproducing the spell, the Wizard records how in ones own spellbook, adapting it to ones own "artistic" style and taste.