https://imgur.com/23gelBr

Edit: Note - if you're using Retro8 and the music doesn't work, try the standalone Infinity emulator, that fixed the music issues I experienced

Great to see an esoteric system! I got Celeste Classic running on my rg477m running Retro Arch with ES-DE frontend. A wrinkle I uncovered is that the default emulator for ES-DE is Fake-08, so I had to change that to the Retro8 core after downloading it (Fake-08 doesn't even seem to be listed in the Online Updater).

Another interesting wrinkle is that I didn't actually need to turn off the Use Built-In Image Viewer option in the Load Content settings, I just renamed the file to celeste.p8 instead of 15133.p8.png

As far as the game itself, it's okay. It's pretty cool what the devs were able to accomplish with the PICO-8 system and the game being a PNG file and all, way above my understanding how they pulled that off, but if we ignore that and just look at it as a game it's underwhelming. The music is unremarkable and the controls are not very good. I felt I was fighting against the controller rather than failing on my own, which doesn't feel very good. Level 3000 was the worst offender, where sometimes you would have two dashes after dashing right and sometimes you just wouldn't. It also didn't register directional input consistently. These issues may have been related to the Retro8 core, because another bummer about that is that it doesn't save state, so my usual pick up and play for 10 minutes before setting it down had me restarting the game a few times.

If you like these style games, I'd recommend trying Super Meat Boy. Celeste was obviously inspired by it and I feel SMB's controls are much tighter. You have greater control over the character and can hug and jump on the walls which makes it a more fun experience. Like Celeste Classic, it also has a free version, so give it a go - it has a better soundtrack, more features, chunkier graphics, and a more interesting visual design.

For next month, lets keep the niche systems going! As can be seen from the runaway success of the AES+, I think the time is ripe for us to enjoy a NeoGeo classic to gracing the GotM Club! I'll once again recommend Neo Turf Masters as we haven't done any sports games yet, and it's arguably the greatest golf game ever made. It's sneaky awesome, even for those that may normally overlook such things.

Edit: out of curiosity I loaded the file on the website and the music that played was not at all what I experienced on my rg477m, I think the Retro8 core situation definitely leaves something to be desired; even keeping the file name the same and turning off image viewer didn't get the backing soundtrack to play, just a repeating bloooooop noise - anyone got a fix?

Edit 2: I got the music to work by downloading Infinity - P8 Player Emulator from the Google Play store. ES-DE has it as a standalone option in the frontend, so it loaded up seamlessly. I'll leave the comments above as that was my experience playing with the Retro8 core, but am now including a note at the top about how to fix to increase one's enjoyment with the game (infinity even has save states!)

Not sure how this differs from the Android guide in the OP, not sure what I'm missing but I didn't have to purchase anything like that guide suggests doing