This is really awesome stuff OP! I like how each monster feels unique and you've nailed it with their abilities and the flavor.
Thank you! That is the most important thing, and the hardest thing, with these first drafts: trying to get their theme / flavor correct. I would definitely appreciate any feedback you can provide
I'll read through the list in a bit more detail over the next week and give you some feedback but one thing I noticed immediately - some of the monsters, Tiamat for example could do with a little reorganisation. Like, thematically I like how the heads can each do something cool but during a big battle, its going to be hard to keep track of 5 spell lists and shes casting 3 spells a round anyway - no ones going to call you out if she casts Cloudkill, Fireball and Lightning Bolt because those all seem like things that Tiamat could do. Keep the bite attack and the breath weapon because those are iconic, but consider decoupling the spells from a specific head and move all the spells to a centralised list to make it easier to run.
Thank you for your thoughts, I look forward to getting some more of your insight. Now, here are my thoughts.
In general I think it is OK for epic threats to be more complex and challenging. They will simply demand more from the DM than the standard monster. They are, after all, epic! I try to give my epic monsters lots of options, but I also try to make them that so they can be run simply if desired. Now, Tiamat is a bit unique and extreme, but she still illustrates some of my primary goals with epic design: 1) capture the them/flavor of the monster, 2) provide options and complexity befitting epic threats, 3) provide a simple combat option, and 4) make them viable solo threats. Let me explain:
1) Tiamat's heads have separate spell list because it ties back to her original design in the 1e MM. To me, it is part of her flavor.
2) Tiamat is a bit more complex than most because of her 5 heads, and this magnified by each head having its own spell list. But, I feel, she should be complex. To fully exploit all she can to a DM needs to really know his/her stuff, including probably having spells pretty well memorized. However, with that being said...
3) Tiamat's spells are extra flavor and not needed at all to run an interesting and exiting battle. They are flavor and there for those DMs who want and can use them, but you could not use them at all and they would not change her CR. Her best damage option (and thus what is important for CR) is always her breath weapon and then her physical attacks. She can be run without spells completely, if someone wants to.
4) In order to be a solo threat Tiamat needs to replace 4-5 monsters to keep PCs occupied. In her case, she partly accomplishes this with her 5 heads. Her stat block is more complex, but I don't think a battle with her is any more complex (and probably less so) than one against 4-5 separate epic monsters.
Now, like I said, Tiamat is a bit extreme because of her 5 heads, so I don't see her getting a lot simpler. She is due for a big over hall soon (once we complete the Great Wyrm dragons) and she will actually be getting a little more complex. The redesign will hearken back to the original Tiamat and provide a separate pool of HP for each head, so they can be knocked out discretely. Thus it will become important that each head will have its own spell list as she will not have access to those spells if that head is knocked out.
So, though I am unlikely to make Tiamat simpler as she is a bit of an outlier, I would really appreciate your feed back. These are 1st drafts, and one of the goals of the 2nd drafts is to simplify them. Thank you for your interest and I look forward to you thoughts and comments.