I've been really lazy to do anything about the room, used both for recording and mixing. Partly because doing anything about 0-150hz really takes so much space. Low end is most difficult to get rid of resonances.
I don't see your point about making recording space less treated. I want it all as dead as possible - not having conflicting ambience to treat in the mix.
My focus right now is to decide about better monitors, with good abilities to adjust to room. Active nearfield monitors with many switches to adjust placement and narrow sweetspot making room less important.
Just as important is your listening position in the room. If in the node or trough in the low end. So some calculations on wavelength and stuff can help out. Just moving around in the room helps too.
I find books really well worth the money. One that really takes a large part talking about listening situation and environment is "Mixing secrets for the small studio" by Mike Senior. Really, really good.