I think you should use the lightning network, I remember that kraken supports it
Discussion
Do you know how can I convert those sats back to btc?
sats are BTC actually
I know that, but how can I transfer them back to on-chain wallet?
Many Lightning wallets let you withdraw on-chain, but you’d likely still incur some fees to do that. I’m pretty sure this is posible with Cash App, Strike, Phoenix and Wallet of Satoshi.
Yes that's true
The only time I ever go from Lightning to on-chain is when I close a channel on my node, but I’ve seen this feature in a bunch of wallets.
If you can use Cash App to receive a Lightning payment, you should be able to withdraw on-chain for free, as long as you meet the (I think) 100K sat minimum.