Ledger/Trezor
Airgapped device (computer/phone)
Steel plate

I dont see any problem with using Electrum because i've been using it for lots of years and never been hacked or really have those loss of coins.
The thing you should really be minding about is on how to store those keys out from other peoples attention and awareness.
Dont store it on a place where it is connected into internet or somewhat.

https://www.youtube.com/watch?v=_VawkuLS3G8