Hey Datfluffypillow - not sure if you've heard from them at all, but I really do hope this is resolved for you soon. I also have to second @Xen's suggestion to not work with people who treat their clients like this in the future, although that may be hard to determine at first glance. I'd personally message them and give them some warning that you're considering filing a chargeback and giving them some time to respond before doing it, but keep that 190 day window in mind. And again, I'm sorry this has happened to you too :(
I will preface this by saying that I am also a very busy college student with a history of long term pain who does commissions when I have the time. I am not perfect and have forgotten to update people before, but I always respond when a commissioner asks me for an update, and the lack of communication is really the biggest issue I'm having. Also, I am a little salty over the unprofessional attitude displayed by their tweet on Twitter. As @Xen puts it, it's deeply unprofessional, and I'm sorry, but my comm was more than 20 dollars, and which is not a small amount of money for a college student.
So I stumbled upon 617th's commissions thread back in 2019. I really liked their sketches so I inquired about a sketchpage comm. They respond with the price, I put in the actual form, and things seem great. They make a post with a list of to-do's in a loose order, update us that they took the weekend off to play some videogames, and then nothing. I DM them on twitter a month or so later to see what's up (my message and their response here: x, x, x). A little while later, they post to TH that they've made their commissions tracking spreadsheet. That helped ease my mind a bit, so I didn't message them for a while.
Quite frankly, I have some anxiety when it comes to reaching out to people concerning commissions. As a result, I didn't really message them until April, when I saw that they took on new commissions. I thought they added a few more to the sheet back in February or so, but I wasn't sure, so I didn't really want to say anything. This time, I receive no response, and they post their tweet a week later asking for sympathy. In my eyes, I have been nothing but polite and sympathetic to them, without seeing any of their alleged updates (I do not follow them on Twitter, and only checked it once in a while).
As of today, I've messaged them one more time saying that it's fine if they have nothing to show me and that I just want to know how they're doing and that I'd just really like some communication, but I don't really have any high hopes.