You will find what you are looking for listed as a USB DAC (Digital to Analog Converter) even though technically, toslink is not analog and you may not need the DAC functionality for your purpose.
There is a long list on amazon. This one claims to be Plug-and-play and need no drivers.
6) How about a 3.5mm male-3.5mm female four pole TRRS (tip-ring-ring-sleeve) extension cord?
The four pole is needed since it's a headset, not just headphones (which only need three pole, tip-ring-sleeve). Examples shown at https://www.google.com/search?psb=1&tbm=shop&q=3.5mm%20headset%20extension%20cable .
Please confirm the plug on the end of ...
Take all the different adapters, switches, and mixers you have, and mount them on a platform. Have a local sheet metal shop make a unifying enclosure, probably power-coated, so all the cables are concealed, and every switch, knob. and button is on the same plane and level. That's what I've done when integrating similar multi-device analog interfaces, to keep ...
Standalone pitch adjuster devices like the TC_Helicon which can adjust pitch to the accuracy you specify are studio devices, designed to fit into a 19" rack mount frame. You'd need to interface them between the audio source and your amplifier, if your amplifier supports that. They will all be more expensive than used PC running tone adjustment apps.