Our first edition games we have been trying a system like this slightly based on the 3.5 spell point system where a level 1 spell is 1 mana, level 2 is 3 mana and level 3 is 5 mana etc. But as you said it had the potential problem of too many higher level spells so we have been trying a split mana for higher level spells. We also gave spontaneous another spell per level usually and restricted prepared to only prepare based on the classes spells per day without adding the attribute. Im not sure how well that would work for 2e yet.
If the lesser and greater mana pools seem like an idea for you to try we do it like this;
1st level spells cost 1 mana.
2nd level spells cost 3 mana.
When you get 3rd level spells they require greater mana instead of lesser, costing 1 for 1.
At 4th level spells, they become the new 1 greater mana and 3rd level now cost 5 lesser instead.
At 5th level spells, 4th cost 7 lesser instead and 5th cost 1 greater.
At 6th level spells, 5ths still cost 1 greater and 6ths cost 2.
At 7th level spells, 5th level are now 9 lesser, 6th spells cost 1 greater and 7th costs 2.
At 8th level spells, 6th level are now 11 lesser, 7th level costs 1 and 8th costs 2.
At 9th level spells we started a 3rd pool costing 1 to 1 legendary mana or what ever you would want to call it and all the earlier spells would cost the same... but as 2e has 10th level spells there would need to be some alterations here as well.
We allowed greater mana to convert to 3 lesser mana, and the level legendary mana to 7 lesser or 2 greater mana.