Hi flamingoMJN,
It is not possible to rule out software conflicts when testing printer drivers, and data may be lost. So, first of all, make sure that you have at least three backup copies of important files (photos, etc.).
I would again try resetting the printing system and re-adding the printer.
Reset the Mac printing system to solve a problem - Apple Support
Add a printer on Mac - Apple Support
Check whether the operating system suggests a suitable driver to use.
I assume that you mean the HP LaserJet 1015 monochrome printer (not an HP Color LaserJet CM1015 or any other 1015 model).
Also, please double-check that you used the https://updates.cdn-apple.com/2020/macos/001-41745-20201210-DBC9B46B-88B2-4032-87D9-449AF1D20804/HewlettPackardPrinterDrivers.dmg download (from the https://support.apple.com/kb/DL1888?viewlocale=en_US&locale=en_US) web page) for the test. An HP LaserJet 1015 driver 2.0.4 is mentioned in the Printer and scanner drivers for Mac - Apple Support article.
If necessary, an IBM information page (https://www.ibm.com/support/pages/information-printers-hp) has further details about the HP LaserJet 1015 printer language/data stream. This printer appears to be somewhat different from the other LaserJet 1010 series models.
You could perhaps check whether a generic PCL driver can be used. I noticed that there is an HP LaserJet 1015 Gutenprint driver as well. This would be experimental. Other experiments may include using the LaserJet 1015 as a shared USB printer in connection with an old Linux (Ubuntu) PC as a CUPS print server. Alternatively, with a Raspberry Pi. There are Linux drivers for the LaserJet 1015 from HP (https://developers.hp.com/hp-linux-imaging-and-printing/supported_devices/index).