Since I'm not able to complete the blockchain synchronization, can I just mine using a pool? And if it's possible, how do I retrieve the coins?
P.S. I made an offline wallet with a Rpi, given that I could not sync.
Yes, if you're mining to a pool, you don't need to sync with the main Ethereum network to mine ETH. Depending on the pool's terms, you'll most likely get paid once you reach a threshold (like 1 ETH) and it's sent to your Ethereum wallet address that you provided when you setup your rig(s). To get your mined rewards, you'll need to sync your wallet to the main Ethereum network. But since it's recorded in the blockchain that you own those rewards (once paid out by the pool), it'll always be on the blockchain and you won't lose it.