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Yes, that's right, I don't trust Trezor, and I certainly don't trust an SPV wallet no matter if it's called Electrum or anything else.
The HD wallet on Core cannot be generated through a seed, but since I don't know the details, I choose to stay under the old non HD format for now.
It's important to not get overly complicated or confused.
Here's the simplest way to create perfectly safe cold storage.
Load some wallet like Electrum or Core. Put some bitcoin in a single adress using this wallet. First a small amount, then a larger, then a larger still.
Verify that the coin is in the blockchain at the address.
Now make a backup of the wallet.dat, the seed words, the passphrases, and triple check that you have them right.
Delete the wallet, the blockchain, and the browser you used to check the blockchain.
You are now done. You have coin on the blockchain, and cold storage of the access methods (s).
Now build another wallet for incidental purchases.
It would be easy to maintain only 1 address in a cold storaged wallet, but this is very bad advice when it comes to maintaining privacy.
Ruling out Electrum, I would need to keep that 1-address wallet.dat file from Bitcoin Core. So everytime I send coins to my cold storage, they all would end up in a single address, which means it could be analized to find out how much BTC you own. You have to avoid this. You don't want a lot of BTC showing up on these "rich addresses" things (I don't have as much by any means, but it is still a mistake to do that privacy wise).
Also if you delete Bitcoin Core you couldn't test if your wallet.dat has been corrupted or not (files can corrupt over time).