@urgendalai here are some notes on helping to find the threshold flow accumulation…
First, keep in mind that the selection is somewhat subjective, not everyone will choose the same value and the one you use doesn’t have to be exact. What you are trying to do is approximate where the first order streams begin.
You can overlay your flow accumulation raster onto various AGO basemaps (such as National Geographic or Topographic) and other stream datasets, such as those you can download from HydroRivers (HydroRIVERS). Here’s a link to high resolution Google satellite basemap that might help, just add it to Pro using Add Data from path.
https://mt.google.com/vt/lyrs=s&hl=en&z={level}&x={col}&y={row}
Not every stream in the FAC will match perfectly with your basemap or other streams datasets, so just use your own judgment. Consider the shape of the channel, does it look natural? Consider the DEM, can you see the cutting into it? It’s hard for me to tell from your maps here, but my initial guess is that the 3000 value is what you should start with. You should make everything below that value transparent so you can see other layers beneath it and play with your symbology until you get a decent match.
I hope that helps.
Nadine