Also... do you know a safe way to find my master public key only using my wallet.dat and my password?
I understand that this is a sensitive subject, but I did not save my master public key when I was first trying this out years ago...
The master public key is only used for the watch-only wallet you don't have full control of your wallet you need to extract the private key from wallet.dat and then sweep/import it to Electrum but it would be much safe if you sweep it to your new Electrum wallet.
To get the private key from your wallet.dat without fully sync you need Bitcoin core installed on your PC and make sure the prune mode is disabled you can check it on the bitcoin core settings to disable it and close it and open it again. Go to help and look for debug window and then go to console tab then type this command below
walletpassphrase "your walletpassphrase here" 600
Make sure you still know the exact password of your wallet.dat
And then send this command to get the private key
dumpprivkey (Bitcoin address you want to export private key)
It will show you the private key and then open your Electrum wallet and import/sweep it there.