So if you have 1548 private keys in a text file, one per each line, you can import this file with Electrum in one go.
Provided that each line has a WIF key and not other format

Is it possible with Bitcoin Core as well?

How about exporting? I want to export all of my keys in one go possibly in one file with one line per key.
Possibly WIF keys but that's not important, i may run a script that converts them to WIF