I have a CID which contains Metadata links to several NFTs (~10000). I want to retrieve this as a json object. I was previously using IPFS Kubo RPC API (https://ipfs.io/api/v0/ls?arg=&json=true) but this gives a 504 Gateway Timeout error for big files. Any suggestions? Can I create a custom API using Helia to achieve the same?
@SoumerMo just FYI, posts at Helia - IPFS Forums are easier for us to find regarding help with Helia 
As far as fetching large content with Helia and converting to JSON, you could totally do that. Check out UnixFS | Documentation
Let us know if you have any further questions