Spell factory needs to be usable during upgrade
tl,dr version: Completely offline is bogus, reduce its efficiency instead
No, this is not a request to make it free or painless. The developers have indicated spells and active play elements are intended to be battle-changing tools. Right now with the spell factory being 100% offline during upgrade this means that your second attack during a clan war will be at a heavy disadvantage.
This disadvantage is also unique compared to the other direct offense buildings: An individual barracks being upgraded is not usable, but the others still are, resulting in barracks overall being 50% to 75% operational; Camps are 100% operational during upgrade.
So my suggestion is this: Make the spell factory only 12.5% (meaning spells take 8x build time) operational during upgrade. This would give most folks enough time to cook spells between first & second clan war attack. If gemmed, the factory would then be 50% operational. This should preserve the value of rushing the upgrade, and create new value when gemming the spell factory.