Why does it say "complete your tax set-up" before going live?

Tax is not needed to go live — you can leave it off if you aren't registered. The message lists what is actually missing: payment and/or shipping.

• Payment — turn on at least one method under Settings → Store Settings → Payment Gateway (Cash on Delivery, bank transfer, UPI or an online gateway all count).
• Shipping — set up a shipping option under Settings → Store Settings → Shipping Setup.

Then click Go live again. Tax still shows as a step in the setup checklist; if you charge GST, set it up under Settings → Store Settings → Manage Tax.