just remember the reason to mantain idle reverse until taxi speed...is better to keep reverse power ready in case of braking fail
Correct. Plus even at idle you are still getting some reverse thrust kepping brake temperatures to a minimum.
Minimum speed for max reverse is 70kts (hence the call during landing roll). I would recommend a smooth reduction from max rev to reach idle by 25kts (assuming your sop is to use max reverse). After that whether you gently slide the TLs into forward idle or snatch them forward beyond idle make little differance to the engines as the FADEC will not give you any engine acceleration unti the reversers have sequenced.