PLAYING FILES VIA USB (MY EXPERIENCE)...Yes!
I can't say for sure as I only had my friend's XH90 to test, but if the media player on the A8 does not offer the ability to advance/delay subs or change their colour/size/placement..., then it's a poor show by Sony. It's a cheap facility which I've had on previous/current TVs (even a 2016 Hisense 4k, Samsung Q9FN & now LG's C9) and I can't think why Sony would omit it.
Playing my collection of films via USB has generally been great with a few caveats as each TV/manufacturer offers different facilities and very often it's almost impossible to find out which through their products' overview:
.the Hisense could play all subtitles (PGS, VobSub, SRT...)
.The Q9FN did not play PGS and also would not play any DTS sound.
.The C9 plays DTS but will not play PGS/VobSub subtitles, only SRT: I believe that the Sony A8 is the same.
Where does that leave you if you need to adjust your Subs or you have sound sync issues?
For years I've been using MkvToolNix
It's free, easy and it does everything you could ever want if you need to adjust the sound or the subs, remove unnecessary soundtracks/subs...
If I have any subtitles missing, on Subscene I have found 99%+ of all the ones I have ever looked for.