Good to hear your thoughts on it - it's always nice to bounce ideas off someone else, especially for a project as big as a homebrew class. Yes, this is coming from experience. Further discussion + Subclasses below:
Re: Lightning Mark vs Stunning Strike, monks also have to land a hit to attempt to stun; just like Markweaver, it's not like they'll spend anything if they don't hit. Your Spell Attack modifier and a monk's attack modifier would be around the same (assuming sane investment), so that's a wash to begin with.
Towards later tiers, Constitution saves become monstrously large, and effects like Legendary Resistance means saves are even harder to stick to enemies. Between that and Monk's notoriously MAD statspread leaving their DCs a bit lower than more specialized classes, Stunning Strike often ends up with a "hit rate" of under 10%; even a level 20 monk blowing all their ki on flurries to get as many Stunning Strike attempts as possible isn't likely to land one per day. (On the other hand, attack rolls are much less resisted mechanically - at worst, there's disadvantage, but melee weapon attacks have plenty of ways to get advantage, and it's not like there's "Legendary AC." Mostly because that'd negate the one thing martials can actually do.
So you land the effect much more frequently than Stunning Strike, but then you have to also have an ally deal damage, so it balances out, right?... No, not really. For one, you usually have 3 allies, so that's thrice as many chances for someone to deal damage - more if you have martials, since damage is pretty much the only thing they do. Even if your party is more magically inclined, save-for-half spells like Fireball and Magic Missile can just guarantee a stun.
And even discounting that, the mark isn't going to be wasted, just delayed - every single turn after is a whole nother set of potential stuns without any further resource expenditure on the part of the Markweaver. And then when the stun does happen, you basically get an actual Stunning Strike attempt at the end of the turn just for icing on top, and it'll be coming off a DC thats liekly higher than Monk's.
Living Tattoo: A lot like the Battlesmith with a custom battle pet. Not entirely a fan of being able to customize its stats, but it doesn't really make a difference except for saves/skills. Two big things worth noting, the temp HP makes you a good amount bulkier (roughly on par with a Fighter), and the Living Tattoo's attacks can be used to trigger your Marks. Mind you, this isn't actually much of an improvement unless your allies aren't able to deal damage for some reason, but it's worth noting. Getting multiple free proficiencies that also ignore your own stats is also really strong, too. Feels like it'd make sense for you to lose benefits like the proficiencies if the tattoo dies though. Being able to move it infinitely far away from yourself is kinda wack, but battlesmith does the same thing so ok I guess. Meaningful Fabrication is a neat noncombat* feature, though at that level your party probably doesnt care about 100g and can brimg whatever in a bag of holding or something.
*Well. Mostly noncombat. I can see a player making an anvil and dropping it on someone, for instance.
Soul warden is neat. Slightly disturbing, you'll have people asking "wait are we the bad guys," but neat. Gives you effectively even more proficiencies, too, just because.
Uncaged Ink... Does it have any use limits or free the trapped creature or anything? Because as written it wounds like you could turn one trapped creature into an army of simulacrae, all active at once.
Changerie Style: Alter Self is neat to have as a base subclass feature i may or may not have tried to pull it off myself with less success, with exploration, social, and combat uses.
Extra Arms doesnt need the clause about shields - they don't stack by default. Though I guess making it clear is good anyways.
The ability to change a willing creature's race... I can think of a lot of ways that could be broken. You probably wouldn't have to be worried unless your players like to make themselves inordinately powerful, though. Custom Lineage + Variant Human + Aarakocra or something, if you want to make things ridiculous.
Your My Canvas, however... Most of the usecases people would go for sound like they'd be broken. Suddenly your DM has to look back at every single encounter the players have had, and a lot of enemy abilities are not intended for players. Pixies, for instance, have Magic Resistance and at-will Greater Invisibility (self-only).
Kinyi Scarification: Scarring Blades is neat, Extra Attack is standard. Might be better to specify specific weapons than to just hope people know what you mean by bladed weapons, just for standard wording. in light of the Living Tattoo, getting the ability to self-trigger Marks at 10th feels a little late.
Memory: I know it's meant more for rp, but even the most noncombatant subclasses like Mastermind Rogue still have combat features as part of their base package. Maybe move the ability to frighten onto the 3rd for unwilling creatures? Or maybe have it be something like "you can imbue your Marks with memories" and have the status effects hook into your Marks instead? Or maybe just have Open Mind be at 3rd; it's kinda like Sun Soul in that it turns a melee class ranged without changing the damage numbers or anything.
