Cookies amongst multiple tenants, subdomains, and custom domains for a SaaS

Hey folks, I'm working on what is ultimately Squarespace for a niche, where users will get a unique subdomain, and can also add their own custom domain (so we have mycoolapp.com where i'll be serving the marketing, john.mycoolapp.com, and maryscoolapp.com).

Looking for guidance on how to get it working with BetterAuth; my stack is SvelteKit, with Postgres + Drizzle (using the direct Postgres connector, though), with Directus as the CMS for end users. Currently I'm having an issue where the cookies are being set on the TLD and not working on the subdomains.

I'm aware that there's an organisation plugin, but that doesn't look to have anything specific to sharing cookies between sites.
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