Sorry I should have made it more clear that for things to show up on OpenSea's mainnet site you would have to deploy your contracts on the Polygon mainnet.

As for the ipfs URI, per OpenSea's docs, it should be `ipfs://CID` :

https://docs.opensea.io/docs/metadata-standards

That said, something I've been thinking about is decentralization through ownership. If you have a custom domain for your own IPFS gateway, you could use that custom domain, because you retain control over the domain even if you move your gateway from self-hosted to hosted and back. If you went down that route, you'd provide the full URL path including your gateway's custom domain. So it would be something like: `CUSTOM_DOMAIN/ipfs/CID`.