A quick google search seems to suggest i have to download the entire 16 years of bitcoin history from bitcoin core that may take hours/days??
Yes, you need to install the bitcoin core, and it must be fully synced because Armory will not work properly if your bitcoin core is not fully synced.
About how many hours or days it depends on your device specs it might take a few days or within a day.
I take it the only alternative is to sweep private keys to electrum?
If you have previously imported the rootkey into Armory, that is a good option. Even though it is offline, you can backup it once more and make sure to select export key lists to get the private keys that you can use to import into Electrum.
You can follow the guide from this link below
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https://bitcointalk.org/index.php?topic=3274219.msg34442580#msg34442580