If your ROM is userdebug, you can use the below commands to remount system partition which will be able to read/write.
adb disable-verity/enable-verit can only be executed if your ROM is userdebug, and you cannot remount system partition if your image is user build.$ adb root $ adb disable-verity $ adb reboot $ adb wait-for-device $ adb remount
If you can flash the image by yourself, you can build eng ROM which eng build will disable verified boot. Alternatively, you can disable verified boot by setting ro.secure to be 0 even if it is userdebug build. You can change the value of ro.secure in [AOSP]/build/core/main.mk, you can refer to the below screenshot.