Huge fan of your guide! I studied it intensively on the wizards forum before it got closed. It's thanks to your guide my party was able to take down a young green dragon at lvl 3 without a single casualty. Can't wait to see what amazing combinations will show up next. Have you considered using true polymorphs secondary effect? That being that if the duration of an hour passes the transformation becomes permanent. Perhaps one could use this to shapeshift into an adult dragon for keeps and use the natural shapeshift ability of its new form/race to maintain its old class abilities and skills?
It won't work for two reasons. The first is because True Polymorph states "The target's game statistics, including mental ability scores, are replaced by the statistics of the new form." That means that all the target's old statistics (its HP, its class levels, its features, its proficiencies, etc.) are replaced by (not added to) the statistics (AKA the stat block, which includes skills, MM 6-8) of the new form.
The second is because, as Change Shape for Adult Gold Dragon states, "In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form , except any class features or legendary actions of that form ." So no matter what a dragon may shapechange into, it doesn't gain new proficiencies, since those would have to replace the old proficiencies. That's not to mention that the Strength score is not among the unaffected Ability Scores when you change shape, meaning that if your interpretation's more liberal than mine, you still have to choose between the Adult Gold Dragon's 29 Strength score and proficiency in Athletics with your new form's Strength score.
So, since it's not possible to become a creature with athletics proficiency unless it already has it, the best Athletics score you're ever going to get by using True Polymorph is that of the CR 13 Storm Giant (+14) which can be matched by a level 13 character with 18 STR and expertise in Athletics.
With the holiday season here and more time-off on my horizon, I'm getting back to the editing process! Finished up the level 3 and 4 spells today and will keep working at this as the month goes on. Let me know if there's anything people want added or addressed!
I'd really like to see your thoughts on the new SCAG content, specifically the Path of the Battlerager for Barbarians. It's got all the bits you seem to like for your grappling builds, what with the 1d4 bonus action attack from Battle Rager Armor and the mobility bonus from Battlerager Charge.