If you are relying on smart phones to gather the GPS info, then one thing you could try for improving accuracy is checking that any power saving options are turned off, as these can reduce the polling rate of the GPS data, and prevent it from updating at all when the device is inactive.
Encouraging people to take multiple images might also help in giving more time for the GPS lock to fine tune itself, so you might at least get a better reading in time for the final image which you could use as the location reference point, or paste it’s data to the other images with the below tool.
In the end you will still be limited by the device accuracy, but you can typically get it to be within ~ 5 metres or so.
I can highly recommend Geosetter for Windows which I have long relied upon for this:
https://geosetter.de/en/
Note that the information on the webpage is outdated and the old v3 version mentioned there is no longer functional, but you can download the latest v4 beta from either the Help → Check for updates menu, or directly from here:
https://geosetter.de/geosetter_beta.exe
You can see the change log or report any issues here:
https://www.geosetter.de/mantis/
Two alternative options that I have not tried myself are GeoTagNinja for Windows:
https://github.com/nemethviktor/GeoTagNinja
And Mappero Geotagger for Linux, macOS and Windows (open source but charges for builds):
https://www.mappero.mardy.it/
https://gitlab.com/mardy/mappero
Apparently you can upload a kml file defining the area desired. See these threads:
https://forum.inaturalist.org/t/how-to-define-geographic-area-for-project/25447
https://forum.inaturalist.org/t/how-to-define-an-area/9540