Draw a core, give each R group its own set of fragments, and every combination is enumerated in the page. A library of thousands is built in the browser, with no upload and no queue.
Then screen it: molecular weight, logP, polar surface area, rotatable bonds and four more predicted properties, plotted so a region of the plot can be brushed and the molecules in it read off.
Draw a core carrying R groups, give each one a set of fragments, and enumerate every product — then screen the library on its predicted properties. The chemistry runs in your browser, so the tool needs JavaScript.