I think 9 spell levels is still too many. Plus the fact that all the existing spells at level 7 and higher, with the lone exception of Wish because it's a sacred cow, do not add anything truly beneficial to the game. I think it's all bad design at those level.
My frustration is with half casters and third casters. I think it's fine to get a spell level 2-3 levels behind a full caster. I think it's honking absurd to get a spell level
6-9 levels behind a full caster. Simply put, there is no way for those new spells to still be the same spells that the other class got and not have them end up completely underpowered. It's ridiculously depressing as a player.
So, what I would do is this:
Level | Heavyweight Caster | Middleweight Caster | Lightweight Caster |
1 | 1 | - | - |
2 | 1 | 1 | - |
3 | 2 | 1 | 1 |
4 | 2 | 2 | 1 |
5 | 3 | 2 | 1 |
6 | 3 | 2 | 2 |
7 | 4 | 3 | 2 |
8 | 4 | 3 | 2 |
9 | 5 | 4 | 3 |
10 | 5 | 4 | 3 |
11 | 6 | 5 | 4 |
12 | 6 | 5 | 4 |
And then I would limit the number of spells per level per day. Heavyweights get up to 4 spells per level per day. Middleweights get 2-3 spells per level per day. Lightweights get 1-2 spells per level per day.
I don't particularly care what happens above level 12, and 5e's design tells me that WotC doesn't really, either.
Multiclassing rules are a tail wagging the dog. The game should, first and foremost, work with single class rules. If rules can't be envisioned that make sense, then
ditch multiclassing.