Enable /system write in TWRP - Systemless root
It appears that the issue was TWRP's fault. To make the /system partition writeable, one could:


Uncheck the checkbox near Mount system partition read-only, in the Mount section of TWRP (screenshot at the end of this answer);
Delete, move or rename the .twrps file to force TWRP's setup wizard to trigger, in order to make /system writeable. This file is located under sdcard/TWRP, and can be managed directly from TWRP's File Manager, accessible from the Advanced section of the recovery. Notice that any TWRP setting customization aside of themes will be reverted.