Subdomain or subdirectory? It looks like a small choice, and it is, until you outgrow whichever one you picked. Here are the things we keep coming back to.

1. Treat your tagging URL like product infrastructure. It is not a marketing toy. Put it on the same uptime monitoring as your checkout.
2. Use a custom domain from day one. Switching later is annoying and almost always loses some history.
3. Hash PII before it leaves your server. SHA-256 the email, normalise it first, and never log the plaintext.
4. Keep a written event taxonomy. It does not need to be elegant. It does need to exist somewhere people can find it.
5. Version your container changes. GTM gives you versions for free. Use them. Future-you will be grateful.
6. Test in preview mode for everything. Including the things you are sure work. Especially those.
7. Reconcile with backend data weekly. A small drift is normal. A growing drift is a fire.
8. Plan for the day you switch providers. Whatever you choose, it should not lock you into a single vendor.

None of these are revolutionary. They are the boring habits that separate teams who trust their data from teams who argue about it. When in doubt, pick the subdomain. It is the choice you will not regret in two years.