Barusu Lamperouge
Headphoneus Supremus
The battery issue is definitely a caveat and only partially ameliorated by going for a budget phone but with a large (5000mah) battery like my Moto. It comes down to the inability to charge and listen at the same time, and I've tried to make that happen using this dongle to no avail. 256GB and larger microSD cards help with the second issue, but that does require one to get a phone with that feature.
I will always maintain that if one likes to carry around a large library, a smartphone is going to be the best way to flip through albums on the go as even an inexpensive smartphone would be more fluid in performance than the fastest Android DAP.
It is definitely a tossup between the convenience of a one-piece solution and the value for money that can be had with dongles and DAC/amps. In the latter case, the bill of materials goes entirely to the sound reproduction bits rather than being split with the interface part. I'm perfectly happy with the Tempotec right now but if I were to upgrade, I'd definitely be looking at something like the ifi Hipdac - more power, more capability and moving up to a real stack
Don't know where you are based, in case you're in Asia i.e. near India and China there are many budget smartphones which have large ass battery (5000maH+) and inbuilt storage of 128gb expandable upto 256gb with good processing power for as little as $300. Also, I agree smartphone does provide best portable library solution as opposed to a DAP from performance standpoint. BT dongles pair up nicely with smartphones and nowadays they are available in all price brackets. Great sound quality with maximum convenience. You should check them out