I'm very curious to take a look at this. Built something similar a few years ago because none of the tax software sufficient accomplished what I needed. Mine takes a list of transactions + transfers, and allows me to audit the balance in each wallet (to spot check and make sure I didn't miss anything). It creates csv exports that I feed into bitcoin.tax and use for tax lot selection.

I'd like to update it to handle optimized tax lot selection, so I'm curious to see how you built that part. Longer term I'd like it to automatically import transactions for me, but haven't made it that far.