Unable to modify file permissions.xml, get error Read-only file system
I was only able to remount rw after 
- removing addonsu (by sideloading in TWRP "su removal" from https://download.lineageos.org/extras)
- installing Magisk (easy, sideloading current release from https://github.com/topjohnwu/Magisk/releases)

I was under the impression that addonsu does not fully grants permissions -- after all I found myself heading down this road when attempting to configure Titanium Backup.

By the way, TWRP has a commandline guide at https://twrp.me/faq/openrecoveryscript.html , all the above is as easy as adb reboot recovery, then adb shell twrp sideload which makes TWRP waiting for a file, so just serve it by e.g. adb shell twrp sideload ~/Downloads/Magisk-v19.3.zip