Yes I'm aware of the spamming problem.
That's one reason I raised concerns over the 20% spread between the character level vs Great Wyrm MRs implied by the table (from 75% success for XVI Great Reds to 95% for XII Great Greens).
To have a 50% chance of one of the attackers' spells penetrating would need an average number of spells of:
75% [Great Red Wyrm]: 2.4 spells
80% [Great Blue Wyrm]: 3.1 spells
85% [Great White Wyrm]: 4.3 spells
90% [Great Black Wyrm]: 6.6 spells
95% [Great Green Wyrm]: 13.5 spells
Assuming the objective is more-or-less uniform result it looks to me that either the Red and Blue Great Wyrms have too low MRs or the Black and Green Great Wyrms have too high MRs.
I'm actually concerned that MR is too high across the board, but I'm not that concerned with the 20% gap, and in particular I'm really not concerned about it if expected rates of penetration increase because the 20% gap is exaggerated by how big 20% is compared to the rate of success. By that I mean a gap between 30% and 50% is pretty trivial, whereas a game between 75% and 95% is fairly huge.
The saving throws absolutely do have to be figured in when trying to figure "number of disintegrates hit by before the target dies". Thus the higher HD dragons have a number of hidden advantages in effective resistance to magic since the higher level party penetrates resistance relatively more often, but the dragon also saves relatively more often. The fact that the saves are also really good is the reason I'm concerned that the magic resistance might be too high across the board.
But the objective is not a 'more or less uniform result'. That objective first of all is a more 4e style objective of fixing the math, and aside from being in my opinion a false goal, requires a system that is not 1e. For example, the 16th level party going up against the great wyrm red, barely has any more hit points than the one going up against the great wyrm white.
Thus, I'm not terribly interested in making breath weapons any more dangerous than they are. As you note, the great white breathes every 2nd round for 35 damage, save half, to basically the whole party. The party probably has 35-85 hit points per character. The great red breathes every second round for 45 damage, save half to basically the whole party. The party six levels later probably has 40-100 hit points per character. But we can't just look at the breath weapon. The non-breath round where the white is physical has an expected damage below 35, depending on how often a 13+ HD creature can expect to hit a 10th level party. The non-breath round where the red is physical has an expected damage below 71 per round, depending on how often a 22+ HD creature can expect to hit a 16th level party. Considering I'd probably be using the 'Isle of Ape' expanded to hit tables, that's probably even slightly more often than the white can, and thus the missing damage from the red's breath weapon has been found, transferred to its bite and claws. Why you might ask? Because the higher level the party, the more likely it is that they can arrange protection against energy attacks. I could try to solve that by accounting for it and making the breath weapon big enough to overcome an expected level of protection, but then I'm back to TPKing any party that can't arrange protection from energy attacks and I'm making 3e type expectations of expected wealth (and with it availability of defenses) by level.
So what I'm interested in is less that everything scales equally, as I am in that damage inflicted over the expected combat is reasonable both from a party perspective an a per figure perspective (total damage is high enough to cause attrition to the party as a whole, but individual damage spikes aren't too high). I suspect just from a theory crafting perspective that this is plenty damage to inflict over the course of however long it takes to kill the dragon for anything less the most munchkinized kitted up post UA fighters in a tactical situation that favors them (cavalier class Paladins with Holy Avengers and girdles of giant strength against a grounded dragon, for example). Still, I'd like that verified against less hypothetical parties, hence my interest in 'play test' reports.
So the gap or ratios don't matter to me that much. I'm not trying to achieve 'balance' in the Diablo derived sense that 4e balanced things. I'm just trying to get into a better ballpark that suits high level play as it probably is within a range of DM assumptions about ability and attribute distribution. It doesn't bother me too much of that level XVI red is a huge challenge to some tables 18th level party, and a push over to some other tables 14th level party. I think in both cases its still likely to be more playable than the stat block it replaces.
Add in the save bonuses in the Dragon revision (Great Wyrms get +6 vs Poison, Illusion, Fear and Enchantment, +3 on attacks that use their Dex adjustment and +2 vs everything else) and most of the 100% MR Great Wyrm's saves only fail on a 1.
Yes. They are bloody well hard to kill and in particular hard to cheese by design. And if anything, maybe too hard to kill with magic. But I can guarantee that no one is going to claim that fighters are completely outclassed by spell-casters.
Incidentally, wands aren't of concern for the more powerful dragons in your proposal. Standard magic wands operate as 6th-level casters so any 1E creature with a MR of 75% or higher is immune to their effects.
The related thread where I talk about good challenges in 1e and why dragons aren't, I also mention that any monster intended to be a significant encounter to a high level party has to be basically immune to wands. Hence the reason I talk about monsters needing at least some sort of magic resistance, just to largely or entirely thwart the caster level of wands. Since 1e AD&D dragons have no magic resistance and limited energy resistance, wands are very effective weapons against them.
