Solid plan. The one thing to flag up front is that the hardest part here is not the automation, it is the social platform APIs. That is where projects like this quietly die, so weight your planning there. Most platforms require app review before you can post on a client's behalf. Meta covers Instagram and Facebook but demands business verification and review, and Instagram only publishes through a Business account linked to a Page. TikTok, LinkedIn and X each have their own hurdles. Budget weeks for approvals, not an afternoon. Your questions in order. OAuth flow, the client approves on the platform, then you exchange the code server side for tokens, never in the frontend since it uses your app secret. Storage, keep tokens encrypted in your database tied to client and platform, Supabase with row level security fits well. Refresh, do not rely on n8n for this in a multi client system, its credential model is built for your own accounts, so own the refresh logic yourself. That answers your backend question too. Put a real backend between the frontend and n8n. Let Supabase Edge Functions or a small Node service own auth, token storage, refresh and client isolation, and hand n8n a valid token at run time. Let n8n do what it is good at, the publish orchestration. Do not make it your whole production backend for a system holding many clients' credentials. For isolation, tag every token, post and log with a client id and filter at the database layer with row level security, so a code bug cannot leak one client's access to another. Biggest lesson, build one platform end to end first, usually Instagram via Meta, get the full approve, token, publish, refresh loop working for a single client, then clone the pattern. Launching six platforms at once means six review processes stalling you in parallel. Worth knowing as a plan B, not your foundation: middleman tools like Blotato give you one API key to publish across all major platforms, with OAuth, refresh and rate limits handled for you, plus official n8n nodes. It skips every app review queue, but you would be renting your core publishing layer and inheriting their pricing, limits and uptime, which for a white label agency system is exactly the part you want to own. Keep it in your back pocket if review timelines block you, but build to own the plumbing if you can.