If it sounds great you aint missing anything regardless of what others say. This hobby is all about the music!
With 480p YouTube video, the audio is compressed, so you might be missing some detail and clarity. For the best quality, consider using a lossless audio source like FLAC or WAV. Your Grado S80 headphones and FiiO DAC/amp combo are solid, but you could explore higher bitrate audio or different headphones for a potentially fuller sound experience.
Yes and no. Would Tidal via roon sound better? Yes. But with entry level (but still very good) gear (grado is good Fiio is good) you have to ask yourself if two $10-15/month subscriptions are worth your while given that after a year at $25 a month you've payed more than your whole setup is worth for acccess to music. There is always "more" there is always better. Even the guy with $6000 Susvara headphones and a $4000 amp/dac can lust after $50,000 Sennheiser HE-1. The question is where do you as an individual feel comfortable and where do you want to stop. The question should be, are you satisfied with what you're doing now, and if you are, it doesn't/shouldn't matter what others think.
