Thanks for the feedback! I actually have a newer version that addresses some of these questions. Thank you for reminding me to post it! Honestly I just forgot.
Anyway to address your specific questions:
1. In the new version you can change spells, but only when you take that evolution again. I made it 1/day for a couple of reasons. First, it more closely matches the way dragon Innate Spellcasting works in the MM. Second, it's more easily balanced against other classes' abilities, many of which are basically "spell 1/day." In a way it's like getting a spell slot, but less flexible. Third, it provides a really easy baseline to balance other evolutions via the spell level (although it's still pretty tricky).
2. The tail attack is also useful for making Opportunity Attacks out to 15 feet, and also for if you want to make bludgeoning attacks. This also reflects the MM, where the tail is largely useless except as legendary actions. It's not even present on the stat block for dragons without legendary actions, but I felt it made more sense to include it (I think it's fine to include class abilities that are flavorful but useless). Making a tail attack instead of a bite attack is certainly balanced if you want to allow that. (But note that v2.4 weakens the multiattack at 11th level by restricting it to claw/claw, which I feel is better balanced in the context of Draconic Might, Elemental Bite and Razor Claws.)
3. I added back in the ASI, and reduced the potency of Draconic Might, and rearranged a few other things. Level 13 now brings a proficiency increase, a breath weapon increase, and a natural armor increase. That may not spark much excitement, but I think that it's fine to have "dead levels" -- trying to force every class to have a special ability at every level can lead to some really scattered and overcomplicated class design. That said, I did try to spread the dragon's abilities so that in general there would be 1 new thing per level.
4. That's definitely by design. Most class abilities that lead to a damage increase for spells are roughly +AbilityMod to damage. Many clerics get +Wis to cantrips, draconic sorcerers get +Cha to spells of one energy type, and when a sorcerer uses Empowered Spell it's effectively the same as +Cha to damage. Since the breath weapon is most like a spell, this feels about right. On a more philosophical note, I'm always hesitant to include a class feature that makes you just plain better at a major feature of the class. For example, if there was a class feature for rogues that made their sneak attacks better, lots of people, craving DPS, would feel "obligated" to take it. With Intense Breath, the damage increase is so marginal that it's easier to justify taking a more interesting power, like a species-specific breath weapon. Of course if a build is focused on breath weapon and wants to eke out every bit of damage it can, Intense Breath can be a good option.
Please let me know how your campaign goes! I think serving as a mount is a super cool angle. My main balance concern is that at low levels, the flight speed and burrow speed might be too strong, but if other party members can ride around on you, that is largely mitigated. ;}
Anyway to address your specific questions:
1. In the new version you can change spells, but only when you take that evolution again. I made it 1/day for a couple of reasons. First, it more closely matches the way dragon Innate Spellcasting works in the MM. Second, it's more easily balanced against other classes' abilities, many of which are basically "spell 1/day." In a way it's like getting a spell slot, but less flexible. Third, it provides a really easy baseline to balance other evolutions via the spell level (although it's still pretty tricky).
2. The tail attack is also useful for making Opportunity Attacks out to 15 feet, and also for if you want to make bludgeoning attacks. This also reflects the MM, where the tail is largely useless except as legendary actions. It's not even present on the stat block for dragons without legendary actions, but I felt it made more sense to include it (I think it's fine to include class abilities that are flavorful but useless). Making a tail attack instead of a bite attack is certainly balanced if you want to allow that. (But note that v2.4 weakens the multiattack at 11th level by restricting it to claw/claw, which I feel is better balanced in the context of Draconic Might, Elemental Bite and Razor Claws.)
3. I added back in the ASI, and reduced the potency of Draconic Might, and rearranged a few other things. Level 13 now brings a proficiency increase, a breath weapon increase, and a natural armor increase. That may not spark much excitement, but I think that it's fine to have "dead levels" -- trying to force every class to have a special ability at every level can lead to some really scattered and overcomplicated class design. That said, I did try to spread the dragon's abilities so that in general there would be 1 new thing per level.
4. That's definitely by design. Most class abilities that lead to a damage increase for spells are roughly +AbilityMod to damage. Many clerics get +Wis to cantrips, draconic sorcerers get +Cha to spells of one energy type, and when a sorcerer uses Empowered Spell it's effectively the same as +Cha to damage. Since the breath weapon is most like a spell, this feels about right. On a more philosophical note, I'm always hesitant to include a class feature that makes you just plain better at a major feature of the class. For example, if there was a class feature for rogues that made their sneak attacks better, lots of people, craving DPS, would feel "obligated" to take it. With Intense Breath, the damage increase is so marginal that it's easier to justify taking a more interesting power, like a species-specific breath weapon. Of course if a build is focused on breath weapon and wants to eke out every bit of damage it can, Intense Breath can be a good option.
Please let me know how your campaign goes! I think serving as a mount is a super cool angle. My main balance concern is that at low levels, the flight speed and burrow speed might be too strong, but if other party members can ride around on you, that is largely mitigated. ;}
