Actually rereading the subclass I’m seeing that others wouldn’t be able to ride around on it like a flying carpet since they’d still be too big until 15th level,
As currently written, a Small creature could ride your flying drake, but you couldn't, at level 7. So, at level 7 two Small drakewardens (Small Harengon, Fairies, Goblins, Kobolds, etc) could summon their medium drakes that can fly, and switch with each other to be able to ride a drake and fly at that level.
So, yes, other people could ride your flying drake at level 7, so long as they're small.
It's just weirdly written, and it gating you from doing the one thing you're known for for no apparent reason.
but the concept of you being able to ride a Medium creature is even weirder to me.
In Icewind Dale: Rime of the Frostmaiden, there's an encounter that is with a Frost Giant (which are Huge) that's mounted on a Mammoth (which is also Huge). This breaks the PHB's rules for mounting, but it's not a big deal, because it's cool.
I definitely wouldn’t go with a flying mount until at least 11th level, and the current 15th level works in two ways: it’s the capstone of an interesting conceptual progression/evolution of a draconic companion who grows and becomes more powerful as you do; it also kinda aligns with the paladin not getting Find Greater Steed to have a flying mount until 13th level.
Here's the thing; those aren't for balance reasons. I don't see any reason why having a flying mount at level 7 for a subclass feature would be mechanically broken. There's clearly more broken things that certain subclasses get access to (again, Twilight Cleric comes to mind, or the Echo Knight being able to summon levitating echoes that can pick them up and move them wherever they want at level 3).
Also, if you already get a flying dragon mount at 7th level, how do you even follow that up in the subclass? What’s the progression after that? The 11th level feature and the 15th level feature without the flight are themselves pretty unimpressive once you’re already a dragon rider. Getting a flying dragon BFF feels very much like a capstone feature.
Uh, by having the drake deal more damage, get access to breath weapons, more movement speeds, magical damage, blindsight (because it is a dragon), an ability to heal itself using its hit dice, or similar abilities. You can easily give impressive capstones and have the subclass be balanced while letting a Dragon Rider ride their dragon (and fly) before late Tier 3 of the campaign.
