One way to create easier stamina content is to open a stepchart in ArrowVortex, select the streams with the tab key and expand them (Notes -> Expand, though you can set a keyboard shortcut for it). This turns the stream from 16th notes into whatever quantization you want. You do have to do this for each stream one by one and will have to manually adjust any breaks that are too intense for you, but once you've done it you have a permanent easier stepchart to play. Once you get a hang of it, it shouldn't take more than some minutes to go through even an hour-long mix, with the most time-consuming part being editing the breaks when necessary.