Chop off the top two levels of spells (8th and 9th) for Wizards, Sorcs, Clerics and Druids
I'd rather start with
1. Use the bard's progression of Clerics, Druids, Sorcerers,and Wizards t
2. Make Clerics and Druid's Spontaneous divine casters (per Unearthed Arcana) severely limiting the number of spells known (or even removing the druid for a Nature based cleric)
3. Limit the Clerics spells known to their deity's domains and a handful of other spells (e..g, Augury, Bestow Curse, Remove Curse, Mark of Justice, Atonement). If you need your cleric's to be healers give them a few cure spells. For Planar Ally spells, the DM creates a specific Planar Ally for each deity. This ally is what is summoned every time.
4. Limiting Wizards and Sorcerers to spells they find.
5. Maybe, make all wizards specialists with tailored spell lists (DMG variant: Spell Lists). I would bring in other specialists wizards from 2e supplements: Alchemist, Artificer, Geomancer, Song Mage, Wild Mage, Dimensionalist, Elementalist, Force Mage, Mentalist, Shadow Mage.
6. Increase the cost and length of time required for crafting potions and scrolls and not give scribe scroll and remove Scribe Scroll as an automatic ability for wizards
7. Ditch natural spell or place more feat pre-requisites.
8. Go through the spell lists, rewrite, re-level and remove spells.
9. Uncouple extra attacks from BAB and make them feats. Give the Fighter, Barbarian, Paladin, and Ranger these feats automatically. No extra attacks for the Cleric or Druid without spending feats.
After that, I would
a. Boost skill points for the fighter and some of the other classes. Add some new skills.
b. Incorporate the Book of Iron Might maneuver system or something similar that allows martial types to inflict ability score penalties, inflict other penalties, daze, stun, knockback, knock prone, temporarily blind or deafen, disable natural attacks, disable certain supernatural attacks (e.g., beholder's eye stalks) with melee and ranged attacks.
c. incorporate a Press the Attack feat that I saw somewhere that allows a character to make an additional five foot step in response to another character (e.g., a spell caster) doing the same.
Again, not a full list of what I would do, but where I would start.