Say I want to meet with someone that sells an item or service, how do I pay this person without revealing your funds?
You will have to reveal some your funds when paying with bitcoin, at least partially.
With right address management and with help of coinjoin, mixing and joinmarket you will significantly reduce the risk of someone knowing everything about your coins and transaction history.
Another solution is to use Lightning Network and second layer solutions like Liquid Network with L-BTC that has confidential transactions, and I think this is perfect for spending account.
2) In order to send the coins into the Android software, you have to transact from Bitcoin Core, into the mixer, then into the Android software address. This is a lot wasted in fees. There is no way to save some money in this process?
You can do the same thing I wrote above, use Lightning and/or Liquid Network and you will save on fees and time.