Revisited this horrible thumpy racket, only this time continued in Reaper after exporting it from FL Studio with "split mixer tracks" ticked, so I can just plonk all the tracks in. Very handy! Obviously still not done, for the most part I've just been adding effects and changing the structure; the mix is still pretty off at points.
I'll probably work on a nicer song once I'm confident enough with Reaper to do more nuanced mixing etc.