I would strongly encourage Opensea to reconsider their decision to deprecate testnet support. L2s are not a solution for the proposed use case. Personally, if marketplaces like Opensea hadn't supported testnets when i started out making art on-chain, i honestly wouldn't have been able to gain the confidence or experience needed to grow my artistic practice to where it is today. There is no better tool and learning environment for on-chain artists than testnets and platforms that support them.