If you haven't added "false" flag after importprivkey, it should automatically do a rescan. If it didn't rescan, you need to since it's necessary to update the balance.
Also, you can also use a blank wallet (non-descriptor) wallet if you do not want to have other private keys in your wallet.

For descriptor wallet, you can try this (to import your whole Electrum wallet): https://bitcointalk.org/index.php?topic=5399337.msg60178631#msg60178631
But instead of "xpub" key, import the "xpriv" key that you can get using the command in Electrum's console: getmasterprivate()
Also, you must add \"active\": true, to the descriptors to be able to derive addresses from them.