MQA through Toslink

That may be a tall order. I checked a couple of low budget DACs  that do MQA, but they don’t have a toslink connector. Of course there may be one, but the $400 budget will limit your selection. You may consider something like the meridian explorer 2 DAC and a toslink to USB or toslink to SPDIF converter.

Can you tell us more about your implementation? Why Toslink? What is the digital source? What is the amplification?  If I have a full picture, I can be of more help :-)

Does the airport express also have the option to output via ethernet and even USB?  This will remove your toslink limitation.  Toslink is not particularly desirable by the way.  Something like the Meridian Explorer 2 can accept a USB signal from the Airport if the airport can use the USB port to output audio.  Usually hifi via ethernet is more desirable.  A previous poster mentioned Bluesound Node 2.  This is a killer option for you.  Forget toslink, go ethernet, and use the Bluesound.  The version 2 is $500 new.  You just have to find a deal on one, which seems possible.  Bluesound has a great control app for android and iOS so you can control with any device and stream Tidal, etc.  

Why not go this way:
computer > network > Airport Ethernet port > Bluesound Node 2 as a streamer and DAC > rca output to amplifier