Nice article. Obvious reason for buying an MP3 player in 2018 - they actually have 3.5mm headphone sockets unlike "smart"-phones which seem hell bent on going full circle and crippling their own functionality to below the level of some +15 year old "Sony Walkman" dumb phones. So if you're like me and just want 1. A phone with decent MP3 player, and 2. Do not care, want or have any social media accounts, then the "functionality" of 2018 smartphones as primarily an MP3 player with a phone attached and long battery life is utterly laughable for the price.
I picked up a Sony NWZ-A15 last year in a Black Friday deal (from £160 down to below £80). Very definitely well worth the money for audio quality & battery life alone. One problem that needs mentioning with ultra-budget £20 players though is stupid artificial restrictions. Eg, the Ruizu X18 you mentioned doesn't list AAC/M4A as being compatible. And many "AGPTek" (and clones) have an absurd but rarely mentioned "maximum 4,000 tracks" firmware limitation which defeats the object of shoving in a larger 128-256GB micro-SD slot into a cheap player the first place. Would be nice to see more mid-range stuff that's both cheaper than the premium Hi-Fi brands, yet more polished than the super-budget stuff.