I may have found a sort-of solution. See attached gif. ASIO4ALL has a control panel (accessible within Reaper audio settings) and I was able to choose the UMC404, etc. and a 256 buffer came up as default. The Reaper settings still say "512" but things seem to work in Reaper with an occasional slight delay at playback start. Recording MIDI keyboard/VST is lovely at 256 buffer setting. However, no other sound source (internet browser, audio player, etc) will emit sound with ASIO chosen -- my choices in Reaper's settings evidently globally control audio for the computer (?). So when I leave Reaper, it seems I must re-set the Reaper audio settings back to DirectSound to have typical PC audio. Good grief