1) Why do I want to use some CEXes?
I took part in some altcoin season 2017 and 2021. First I lost most of BTC I bought in 2016, then I made some progress, having profit in 2021 (still made a lot of mistakes). I wanted to try to repeat it in 2025 and unfortunately some coins are only available on CEX.
I've also taken part in the altcoin season back in 2017. I've made a decent profit from it notwithstanding my countless mistakes. I don't know if such season would repeat. It seems the kind of altcoin season nowadays is niche-based. Meme coins have their own bull runs. NFTs have theirs as well. AI coins have also become more popular. They don't seem to move as one. Back in 2017, I cannot remember such compartmentalized altcoin bull runs.
Anyway, if this is your plan, you might end up having accounts on different CEXes.
2) Regarding proof of funds, my basic investment was in 2016, when Bitcoin was extremely cheap, so I did not have to invest big money to buy first bitcoins. However on my addresses last transactions from exchanges are from 2021-22 and if someone asks about sources of fund that time I will probably not able to prove it.
Since that time I am active in ETH and L2 and here my whole history is quite transparent on blockchain, but finally I want to move some BTC to altcoins soon. Maybe it is not worth to lose privacy, I don' t know. I heard there are some decentralized method for transferring BTC to other networks, but can you recommend something really safe and cost-effective?
I remember I was asked for proof of funds once or twice, but it was enough to provide bank statements each time. I'm sure you have bank statements as well.
I cannot remember it completely but I guess I have read of a particular case in which the user was asked of something like proof of acquisition. This is kind of hard to prove especially if you've been a hodler for many years, but I think this seldom happens.
"Transferring BTC to other networks"? I'm afraid there's no such thing technically. BTC stays only on the Bitcoin network. But there are cross-chain bridges. You could, for example, make use of your BTC in the Ethereum ecosystem by using a Bitcoin-Ethereum bridge, but what you will have in the process isn't actually BTC but an Ethereum token. You're provided with Wrapped BTC (WBTC). There's also BTC Token if you want to use your BTC on the Tron network. You can also use cross-chain bridges to take advantage of these BTC-pegged tokens in other ecosystems.
As to their safety, that's not guaranteed. Many successful hacks are recorded in cross-chain bridges, and for several reasons including smart contract vulnerabilities. As to their cost-effectiveness, I doubt it. It might be costly, although there are so many options available, but you might need to jump from one bridge to another.