I researched this a little further and discovered in the SafetyWing terms and conditions that their cap on payouts is quite low ($250k). Actually their underwriting company appears to be Tokio Marine and a larger coverage is available through their website, with more options for customising your package.
But in the end I went with World Nomads Travel Insurance ( https://www.worldnomads.com/ ) where I found a competitive premium, underwritten by AXA, for unlimited coverage.