Assuming Device=Computer, i think this is more or less what i was doing.
I assume encryption passphrase = electrum password
So that the Electrum data files in the offline computer is encrypted
Would be interested to learn how. I guess i would have to download the whole blockchain, would't i?
You've reasoned well,
Just download Bitcoin Core and make sure you have an SSD (HDD will also work, but I recommend using SSD) with a capacity of at least 1TB or 2TB ~ currently the Bitcoin network weighs around 700GB (I haven't checked it for a while).
Optionally, you can enable pruning so you don't need to keep the entire blockchain stored and the node will only download and check the recent blocks and delete the old ones, you can set the size manually.