Converting road single speed bike to 1x
Steel frame can be set out to accommodate a 135 mm rear hub without substantial effort (bear in mind that the dropouts will need to be realigned).
With a rear derailleur the chainline is not an issue (in my opinion) as long as it is not kept on the most outside cogs in the rear.

You can also play with the chainline by mounting another crankset (different offset) or changing the bottom bracket spindle (you mention that it is threadless, you don't say which type is it exactly, cartridge bearings vs cup and cone).

What is often an issue in such conversion (I personally dropped one frame and kept it as fixie because of that) is whether all the derailleur cable eyelets and shifter bosses are available. Also you'd need the derailleur hanger or a suitable adapter.

Have you considered internal gearing hubs? There are Sturmey-Archer or Shimano models widely available with up to 8 gears if I'm not mistaken.