Not sure what exactly you want, but you can derive millions of Bitcoin addresses from one master public key. Check iancoleman to see how it works.

Loading the balances of millions of addresses won't work if you rely on third parties. You'll have to run your own Bitcoin node for sure. If you're keen on using electrum here's a nice way to achieve what you want:  Electrum Personal Server.

Can you analyze the first reason a little bit more?

Besides the above, he'll expose all these addresses to electrum nodes, which isn't recommended for privacy reasons, especially if they're clients' addresses.