there is I believe only one way left through which someone could figure out the addresses are connected: the fact that Electrum communicates with other servers to figure out the balance of your addresses.
There are a couple of others I can think of.
If you ever give out your master public key.
If you give multiple addresses to the same service while logged in under the same account (for example, an exchange to process withdrawals), then the exchange and all the third parties they share information with will know all the addresses belong to you, even if you haven't linked them from a blockchain point of view.
If you use the same unusual script on all the addresses, then that gives a pretty strong indication they all belong to the same person.
If you were to make two different transactions from two addresses a few minutes apart, and both transactions pay from the same address type, pay to the same address type, pay the same fee rate, use the same nLockTime, the same nSequence, the same version, etc. then although not definitive by any means, it might cause a focused attacker to keep an eye on those addresses.