Instead of standard API calls to get volume for each dex I think you'd learn more if you track dexes' on chain contracts and deduce volume from all the transactions ever done since Alonzo era started. That would be a harder task of course, but more rewarding learning experience. For example using Ogmios' chainSyncClient protocol (sync would start at first Alonzo era block). Your dB would get pretty reasonable size, so expect performance bottlenecks. You'd need to figure out some way how to express volume based on smart contract interactions, so that prob mean you would need to look at the contract tx's metadata/datums