expansion is important to me as well. Perhaps we're both old school but I also understand the use of 2 DAC chips and defer to IBasso here. Also like proven technologies. Hard to know what you don't know.
S-Pro master is just a class D output amp with all the good and bad that can come with it. Class D amps aren't generally favored for home use but it seems they got it pretty right for this purpose. Best one I heard for home use had a linear power supply and good size output chokes. Not things that we'll see here. They are very efficient but Sony haven't reinvented the wheel. They made their 1st class D amplifier almost 40 years ago. It's not new tech.
Makers like FiiO and IBasso can choose from a variety of devices and proven tech. Even using output coupling caps has certain advantages over active dc cancelling. Everything is trade offs, including the Super Duper S-Pro Master Blaster amplifier. Sorry, it's not a dig of the device, just the marketing terminology to be bantered about by fans.
I don't think we should tell others what storage type will suffice just because it does for you. With 24/192 files 128gb will you'll get about 60 albums. Try streaming 24/192 wirelessly. It may not work as well as you suppose and you're now carrying 2 devices around.
That said, I think it's great that you found your personal perfect DAP.