What is so unreasonable about ten thousand men taking high garden? The dornish sacked it during he gardener civil war and I doubt the dornish king who did that had more then ten thousand men

Because in that case there was an actual civil war, plus simultaneous invasions by the King of the Rock and the Storm King, plus another Dornish raid on Oldtown. 

Here, there’s no crisis dividing Reach forces – one modestly-sized army marches unimpeded across a seemingly empty Reach and takes a castle that should have had tens and tens of thousands of troops protecting it, given that Olenna had been in the process of mobilizing for war since the previous season.