SPDIF_OUT to TOSLINK backplate arrived today, connected to mobo GND,SPD,5V then hooked up to receiver via optical cable, booted EOS & set Built-In Audio Profile to Digital Surround 5.1 (IEC958/AC3) Output in Volume Control > Configure.
(near) Instant Dolby Digital 3/2 surround playback, tested working in Netflix (via Brave).
Yet to test surround audio in games via Steam etc et al.