A one day Travelcard is definitely an option but you may have to put in a journey where it’s the cheaper option to get it.
I think I put a journey in from Slade Green to Upminster returning the same day and I got it no worries.
Just looked at my expired tickets and it says Slade Green to London underground z1-6 as my journey
Was same price as Travelcard and I used it all day without issues.
Interestingly, I discovered that despite Southeastern running commuter services into London, you can’t buy an inboundary Day Travelcard on their “key smartcard”