The optical out on your TV might be only meant for its internal DVB-S receiver (if it has one). In a little bit I doubt, that a conversion from HDMI to some optical out (Toslink or S/PDIF) will work in that way using external devices.It could probably send the 2 channels to your amp where you might get DolbyProLogic IIx or something like that. But that´s 2ch PCM then and not a conversion from a bitstream coming from HDMI to some optical out.
ARC is something different which is also able to handle external devices connected via HDMI. But ARC also has to be supported by your AVR. I nearly have the same problem. My AVR also isn´t ARC-capable and if I ever get a 4kTV I have to make my thoughts how to deal with that.
In my case I probably consider some HDMI spliiter (1 in 2 out). So I´m able to send the audio to the AVR and the video to the TV directly.