I have previously used apps to track my expenses, but I wasn't consistent with manually updating it. Is there a way to extract purchases logs from Apple Wallet? Or is there an app that automates logging my expenses?
This is one of the main features of the Apple Card. It tracks your spending and is integrated into Wallet.
If you're asking about 3rd party cards stored in wallet, I'm pretty sure the answer is no, although there are apps like Mint that will track and categorize your transaction history (but they don't get that data from Apple Wallet)