@danjacksonuk

Good ideas, thank you. There is a German Telegram group is already available. Feel free to create a group in English and paste the link to get in here.

We do our best to answer all questions appear in reasonable time.

Regarding your topic. We do not recommend to track FIAT deposits and withdrawals at all. It is described here: Different ways of managing your portfolio (https://cointracking.freshdesk.com/a/solutions/articles/29000018265/en)
The calculation of the gain is always sell value minus buy value. You spend "just" 8k and not 10k so far according your example.
Buy value of 4000 USD (2 ETH * 2000 USD - the ones left does not matter --> listed under "unrealized gain" as you did not sell them so far) minus 10000 USD (2 ETH * 5000 USD) which leads to 6000 USD gain (10000-4000). And you have your 2 ETH left which you have not sold yet.
See How does the CoinTracking purchase pool work? (https://cointracking.freshdesk.com/a/solutions/articles/29000031793?lang=en)