Updating my online wallet to not rely on the Blockchain.info API
I do not want to use third party services or APIs like Blockchain o
  Coinbase. I do not want to download and process all the Blockchain.


If you don't want to use 3rd party APIs, you should have your own node which requires downloaded the full blockchain. There's blockchain torrent https://bitcoin.org/bin/block-chain/ that speeds up the download process, and there's advanced Bitcoin node libraries like http://bitcore.io that integrate with Bitcoin core.

There's more reliable blockchain as a service like BlockCypher https://dev.blockcypher.com which is programming language agnostic. I use it all the time and I am satisfied.