After days of frustration due to Poly not performing as expected (iPhone X+Glider), I did some troubleshooting with
@joe28 who advised me to reach out to Chord.
As a measure of last resort, I tried my third and last SD card (of similar spec to the first two) and, somehow, everything improved immensely. Playback with Glider is now smooth as butter, irregardless of the type of file being used- FLAC, DSD64 or other (this is on Poly's internal hotspot);
I still have not tested things on my home wifi, where I was getting problems, though. I also expect that I will still get track not available errors on Roon too. However, for the first time ever, I can enjoy my music in stunning quality, on the go and without interruptions. Some small niggles remain- like having to reboot Mojopoly if I have used Airplay before starting Glider and the GoFigure app still nowhere to be seen. I am, also for the first time, pretty sure that there is a bright future ahead for all of us early adopters.