I would manually verify top winners by other means (e.g. finish photo).
For mass fraud, I think New York Road Runners (NYRR) has quietly solved this.
Uploading a manual activity gives me a 9+1 credit for $20? Seems like a win-win - more people fraud, more the non-profit makes.
I don't work for Strava so don't know what they are / could do to block fake activities but is there an option to require enhanced fact checking for manual uploads?
A friend of mine had a watch that doesn't sync to Strava and the steps to upload manual were too laborious so they changed the device. However, the watch app did did display a map of the route and this would also be available on (say) an unlinked Garmin (other watches are available) account; could you require manual uploads to add screenshots of the source data?
I'm sure there will be ways to fake on two sites and some technology (I've an old handheld) that is less linked to "platforms" but it will be more complex to achieve this.
Or cross check that the winning activity is not the only manual upload in a profile where everything else on a profile is from something automated?
Or say "really sorry but manual uploads don't count" (I've seen this for some sponsored challenges, I think!)
I don't like the idea of excluding people from things because their kit is not the latest/best/most integrated but it's reasonable to set a bar to prove an activity is real if someone else is going to miss out on a prize.
Elsewhere I've suggested a long challenge/competition/leaderboard ban and prominent red flag for users who are shown to have deliberately faked activities...