Completely ignored the actual post. Oof, sorry.
This will be very technical: If you go single speed you will need a way to tension the chain whether that be a device made to replace the derailleur and act as tension alone or jist using the old derailleur with the limit screws set so it can only be on the single cog. Or even just moving the wheel fore and aft in horizontal drop outs.
Easiest way to get your multispeed down to one is take the extra chainrings off the front (be it a 2x9 or 3x6) and leave the rear in the cog you like the best. Don't need to run any cables to the back just play with the limit screws.
I think ditching the front derailleur does makes sense here, leave more room for the engine but pictures will help immensely.
If your bike has a thread on freewheel you can just get a single speed freewheel, then re-dish the wheel to get a good chainline or if its a freehub with a cassette you can get spacers and only leave one on.