Since your going to do so many dragons, I'll just note a few of my desires and pet peeves about the standard dragons.
To me, Dragons are supposed to be Alpha Predators, top dogs on the planetary food chain (and of course the namesake of the franchise). They are the embodiment of the living tank....they combine a strong durable defense with an incredible offense. Its not that dragons are the toughest things, and its not that they are the most damaging things....its that they combine high strengths in both to generate a weapon you just do not mess with.
So a few notes:
Generally I agree, but I just want to point out that I am going with the standard D&D trope that dragons get stronger as they age. They are only "Legendary" when the get to Adult and after that they get more more versatile and powerful. So anything younger than Adult will not really be "Alpha Predator," but getting there!
1) Dragons as written have incredible senses and even get detect as a legendary action. But it doesn't do anything, its just perception. I wish this was pushed further, dragons don't just see you....they see "through you". They see your soul, your thoughts, your intentions. This could be manifested as a form of foresight, advantage against foes they use detect on, able to exploit a vulnerability (aka vulnerability the game mechanic) you didn't even know you had, etc.
Very good idea, I will make use of that!
2) Dragon breath weapons are their hallmark, and though they do good damage, they aren't always the sexiest option. I like to see breath's really scale up on the higher dragons, they can burn away magics, alter their shapes, linger and shape the battlefield, recharge more often at high levels, etc.
I already do some of this with my dragons. The trick is finding the right balance. The issue you have to be careful with is the affect on CR as the breathweapon plays a lot into that, so expecting to be used more often means everything else is weaker. Now, I have given my dragons a legendary recharge action before. Spend legendary actions to recharge your breathweapon, but use it can balance things out.
3) Dragon defenses should always be tough....and the high level dragons are I think worthy of true Damage Reduction (similar to the heavy armor mastery feat, but against magic weapons too). Its not a damage threshold, but true honest to god damage reduction. Also top dragons should get proficiencies in all saves, dragons don't have weak points to exploit like lesser monsters, they are always tough, its just some things are tougher than others. That said, I wouldn't not give them magic resistance, as dragons are innate creatures of magic, and frankly that mechanic is used way too often for higher CR monsters. Though of course, some kind of "unstoppable" ability as you have already used in other monsters.
I'm already improving saves for monsters generally, and agree with you on dragons getting them all. Not sure about true damage reduction. We use it in our games, but it doesn't seem like it gets much traction with the general gaming audience. Messes with CR calculations too. I'll think about it.
Regarding magic resistance, you used a double-negative implying you would keep magic resistance, but it seems you want to be removed? Or are you suggesting it be removed from other monsters? I've used Limited Magic Resistance before (advantage on spells up to a certain level). MR hurts CR (+2 to effective AC) so getting rid of it would allow more design space. With high saves dragons would be good mostly without it except for Dex. Maybe they have something similar to "Evasion" as that only adds +1 to AC.
Dragon Scales. The dragon is immune to all damage, except psychic damage, unless the attack or effect inflicts more than 10 hit points of damage, in which case it takes damage as normal. Additionally, if the dragon is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.
4) I like that dragons are magical creatures and so get magical effects as they age, but spellcasting never feels right for them. I would love to see some magics imbued into their abilities. They have mage armor on all the time (reflected in their AC), some dragons breath blows you away like gust of wind (included in the breath stat block), some essence dragons have a passwall innate abiity that they can literally just pass through walls and barriers. So just ideas, but the thought being is that dragons should not "cast" spells in a fight, magic is an innate part of them, they are magic, so its either always on, or always applied to their attacks and movements.
I agree in general, but it gets unwieldy in a stat block quickly. I typically give them some themed magical things (red dragons are good with fire so I give them some fire themed magic), but don't go overboard. What I think might work is to give them innate abilities similar to magic items, like their bite is similar to a sword of sharpness or something similar. Maybe that is what you mean by, "imbued into their abilities."
I also tend to give older dragons access to lair actions, without being in their lair. I am think lair actions should scale as the dragon ages as well.