Thank you, nerioseole, for another enlightning reply.  You even answered my next question in your edit - I am considering putting a full node in the cloud, and then use that as my own electrum server.   I was under the impression that Electrum could also fall back to SPV mode if there are no servers, but maybe I am mistaken.

I will definitely consider using a Trezor.  I have been a bit reluctant since I am not sure what happens if it breaks and they go out of business, but I presume the private keys can be recovered from the seed without a Trezor.  I also have to convince myself that somebody getting hold of both my laptop (with the watch-only wallet) and the Trezor cannot spend the funds.

Otherwise, I will probably do the SSSS trick with the electrum seed on an offline raspberry pi, and use that as the offline wallet.  I am aware of the theoretical risk of transferring malware between my laptop and the R-Pi over the USB stick (this nasty hardware exploit people are talking about), but that almost seems to require somebody targetting my setup specifically, and I do not believe I have near enough BTC for that to be worth the effort

SSSS in electrum would be nice.  Alan Reiner from Armory always says that one's heirs should be able to recover the funds - that will be a lot easier if all they have to do is to copy the three parts of the key into Electrum