I see a lot of people mentioning this here, but this is not always possible. The most obvious reason, anyone participating in a signature campaign is receiving repeated payments on the same address for the duration of the campaign. If you now use these funds to pay someone, this person could not only see the max amount of coins you have there currently or the maximum accounts you held there, but they could just google the address and find out your profile and ultimately source of funds.
Receiving funds in the same address from a signature campaign is the most common option. However, even in the scenario that you mention, if you (as an entity) are well hidden behind the "takuma sato" persona, how will the person that you pay know who you are in real life? And, at the same time, why should they care?
Anyway, your point is important, so I get it. One user here (paid2 - I mention the name since he discussed it openly in another thread) does something clever, which is a good option to consider. I saw that BlackHatCoiner mentioned it above too. You can use a non-kyc exchange, like the one in my signature and exchange all the funds from your address into XMR. Then you can use the same exchange or any other that you want to exchange from XMR back to BTC (obviously using another BTC address). Don't forget to use Tor for this. You will make it impossible for the payee to imply that "takuma sato received X payments from a signature campaign and then used these funds to pay me".