As a later project you could consider an output module that takes two or more inputs, mixes and pans them into a stereo field, attenuates them to line level, and sends them to two mono output jacks (or a stereo one, but two mono is more versatile). Maybe also via a headphone amplifier to a headphone jack. There are a few such designs around — I built one, though the mechanical design was so stupid I haven’t published it.