I understand that the Electrum server is *between* the Electrum client (=Wallet) and Bitcoin node.
That's true.
In fact, the Electrum server, after asking Bitcoin Core, fulfill the Electrum client request.
That's generally true.
Due to the decentralized nature of the blockchain, a full node is like a local copy of the blockchain. Am i right?
Node also expected to verify and share blockchain with other connected nodes.
Still, and although it might not be safe, would it possible to tell the Electrum server to rely on a Bitcoin node hosted remotely by somebody that i trust?
It's impossible. Besides, how would you know whether Electrum server actually connect to specific Bitcoin node.
I know it might not be very good for the safety of the blockchain (if everybody does that the system becomes centralized), In other words is it possible for several Electrum servers to use the same Bitcoin node?
It's theoretically possible. But Electrum server connect to Bitcoin Core to using RPC-JSON protocol which require authentication (either username/password or cookie).
Also, is the only benefit of running (hosting) a Bitcoin node and an Electrum server is to avoid public servers (therefore ran by strangers) to associate a wallet with an IP?
Yeah, it's common reason why people run their own Electrum server. Although Electrum server may be used if you also run your own block explorer or developer who need to get list of transaction related with certain Bitcoin address.
Also since my IP has already been associated to the btc transactions already made, don't you think it is too late? Only future transactions, and if never rely on a public server again would be private.
It's "better late than never". And regarding this matter, some member may suggest you to move your Bitcoin to new wallet (which have different list of address). Although you need to be very careful since moving Bitcoin from multiple address may reduce your privacy further since it'll also leave trace on blockchain.