So you deposit this money in an exchange: You already have problems explaining the origin of the funds. The funds may or not be accepted. Then if you ever got out of that situation, you would need to prove authorities source of funds to the last sat. The fact that they don't have KYC is already a problem, but since this was ages ago you did not keep 100% of records, some websites are missing, etc. You also advertised mixers, so it's questionable how they would react.
But if you want to go this way, depending on the country where you live, you can simply declare that you got paid for marketing etc., so you can just pay taxes on this income.
I don't really understand the pickle, unless you live in a country where everything related with bitcoin (holding, earning, buying) is illegal, in which case, I don't think we should be having this conversation at all.
This is unfortunately not the case in many countries. If you look at EU for instance, they really hate cryptos by default. If you are selling from a known exchange where you are fully KYC, you may be able to cash out, assuming you are in good standings with your bank account, and you have proof of funds, which means you will have to provide buy orders and sell orders on reputable exchanges, as well as transaction history from your bank account to the exchange. This is the standard, and what the taxman and banks expect.
Now the moment you want to cash out for instance, signature campaign earnings, it becomes more complex, there's no way to know how they will react to that, either the taxman or the guy doing compliance at the bank, so you are gambling essentially, with what the outcome will be. You are getting paid by anonymous people behind a nickname. When you do business, they will expect to have an identifiable tax number with your business partners, here you get paid by random people. Now add in if you lack some tracing in your history, which chances are you are if you started 10 years ago or so, and the tracking becomes complex. So if you start cashing out chunks, you are going to have a problem. They will ask where this money is coming from, so you would need to disclose your entire wallet, meaning that you better have a good audit.
It's not fair that people get treated like this, but it is what it is. The truth is, cashing out is not as easy unless you got it from KYC sources were everyone is identified. The easiest non-KYC situation is a miner that has freshly minted coins since that would be easy to prove on the blockchain. But the moment the coins move around through non-KYC parties and back into the fiat circuit it becomes difficult, and so you want to really know what you are doing before you take any steps.