On that topic, I accidentally just bought b/w toner for 18k pages for 70 € yesterday (I thought it was for 6k pages). https://www.ebay.de/itm/194967229480
I bought third-party toner that came with a faulty chip, transplanted an original brother chip to it, and used the reset menu (open the printer to gain toner access, hold ← and X for 2 seconds, press ←, select which toner chip to reset as if it was full) and now it's working fine :)