Staking + Purchasing/Trading in one wallet

In my understanding I can delegate any amount of ADA in a wallet for staking. I do not necessarily need to stake the whole amount of this account. If this is correct then there is an interesting question: Do the staked ADA alway stay the exact ADA I delegated for staking or is just the amount of ADA fixed? For some of us this could make a huge difference. In Germany e.g. up to now crypto gains are tax free if you hold your coins for longer than one year. They use the FIFO (first in, first out) method to decide which coins were sold. So just let’s make an example: You have a wallet with 10.000 ADA of which you stake 9.000 and you use 1.000 for purchases and trading. You expect to trade 5 times per year. Then we can have two possibilities:

1.) The staked ADAs are exactly defined and alway stay the same:
Here only the coins 9.001 - 10.000 are used for trading, they leave and enter the wallet several times per year. In Germany you have to pay taxes for your trading gains.

2.) The staked ADAs are not exactly defined, the staked amount in the wallet is just locked so that this account cannot fall below the staked amount.
In this case all 10.000 ADA would be used for the FIFO calculation and trading five times per year with 1000 ADA each would lead to a situation where each ADA stays for more than a year in the wallet so the trading would be tax free.

Does anyone know if it’s possibility 1 or 2 (or maybe 3…)?

Where does the reward account fit?

Sorry, I don’t understand your question. Can you explain?

I think there’s an account associated with your registered staking key, where your rewards are deposited for withdrawals after an epoch if you wish. If you decide not to withdraw will it still be counted in your calculation? And you might even use the reward account for staking.

I just tried to show a simple example because I think this way it’s easier to understand what I want to express. I didn’t include staking rewards but I’m sure, they don’t change the game, it will be maybe 1 or 2% per year of your staking.