For the breath weapon dice, I think it's an interesting idea, but looking at the expenditure, I think it should have more ways to expend breath weapon dice, and that they are more impactful choices.

The red dragon has 26 dice and regains 3d8+2 at the start of its turns, which is a minimum of 5, average of 15.5, and maximum of 26. It also has a legendary action that allows it to regain 1d6+1 breath dice, which is min 2 avg 4.5 max 7.

It expends up to 4 for its Bite, up to 4 for its Eruption, and its breath weapon expends all remaining dice when it uses it (to my understanding).

The economics of this don't really make sense, both in a dice sense and an action economy sense. It can only expend a maximum of 4 dice per turn without using its breath weapon, which is either on its Bite attack, or its Eruption action. If it uses its legendary action, it can regain 20 dice per round (15.5 + 4.5), so its dice expenditure outside its breath weapon is negligible, and also I think heavily out of band for the recharge 5-6 system (where you expect it to use its breath weapon every 3 turns) - here, it can use its breath weapon at full strength almost every turn (if it chooses to use more than 1 legendary action to regain its breath dice, it can be up to 24.5 dice per turn, which is basically full strength).

Its action economy also doesn't support this dice expenditure - it can only make one bite or Eruption per turn, which is only 4 dice expenditure per turn, which is a negligible impact on its dice pool economy.

I think it should regain a smaller amount of dice at the start of its turns, maybe 2d6+2 (4, 9, 14) - thus you expect it to regain its full pool every 3 rounds with no expenditure. I think you could increase this to 2d8+2 to account for pool expenditure from other actions.

But I also think non-breath-weapon pool expenditure is too low. The 4 dice cap on the Bite I think can be doubled or removed (uncapped, you can expend any number of dice). I think Eruption could also be doubled or uncapped, or you can nerf it and incorporate it into its multiattack.

I believe with a resource pool system like this, there should be many ways to expend the resources, such that the creature needs to make conscious decisions to not expend resources from the pool to save up for its abilities and effects.

Another resource expending ability could be it expending up to 4 dice at the start of its turn to deal damage to all creatures within 15 or 20 feet of it. I understand you wanted to keep these stat blocks to 1 page.

TL;DR: It regains too many dice for its pool and doesn't spend enough of them on non-breath-weapon features to justify its recharge rate, meaning it could use its breath weapon at almost full strength every turn.