The root of the problem you had with Bitcoin Core is the "pruning" setting which avoids you to swap wallet.dat files entirely.
Plus replacing the file without shutting down Core will result with that seemingly successful (but improper) import of wallet.dat.

Electrum on the other hand is an "SPV" (Simplified Payment Verification) wallet which doesn't rely on a local copy of the blockchain.
Your wallet will only connect to the public servers for balance checking,
so you can add keys and get the correct balance without waiting for days/hours nor will encounter issues like you have had.