I have tried some more papers.
The Strathmore 500 bristol 1ply I got was very alkaline and pretreatment with citric or oxalic acid ruined the smooth surface but still did not give a decent print.
Ruscombe Herschel is much harder than Buxton but has significant more surface structure which is quite distracting. A smooth plate version would be very interesting especially for smaller prints.
One Paper which is quite promising is Zerkall book smooth white. It is slightly alkaline but seem to work with some oxalic acid added to the ferric oxalate. It might be the second best to Buxton in tonality but because of its smoother surface the print are much sharper.