@OP electrum seeds are not bip39 and can't be restored in most other wallets.
For this reason alone I prefer to not use Electrum seeds for cold storage. A Bip-39 seed for cold storage is more practical, in my opinion, since it gives you more options to restore the wallet in case of an emergency or lack of access to computers or the Electrum software. All you would need is a hardware wallet or a wide variety of wallet software clients to restore a Bip-39 seed.
Electrum seeds are great if you plan to use it as a hot wallet. They are designed with an additional checksum to inform the client whether the seed is intended for a segwit wallet or a legacy wallet, helping the user when restoring the wallet from seed.
I've used the Ian Coleman tool to generate 24-word seeds, but I agree with others who've raised concerns about Java and browser-based entropy generators, they don't instill much confidence. The Ian Coleman tool does allow one to enter their own entropy, so I use /dev/urandom to generate 256-bit entropy that I then enter into the Ian Coleman tool.
This command will generate a 256-bit HEX string that can be used for entropy:
cat /dev/urandom | tr -dc 'A-F0-9' | fold -w 256 | head -n 1