Should work but probably not the screensaver function (see: https://community.volumio.com/t/waveshare-5-5-amoled/41787).
On a Raspberry Pi there is no “config.sys”, but I assume you meant “config.txt” (?). If your Volumio image should be based on at least version 2.673 I would recommend to add the display specific settings
max_framebuffer_height=1920
config_hdmi_boost=10
hdmi_group=2
hdmi_mode=87
hdmi_timings=1080 1 26 4 50 1920 1 8 2 6 0 0 0 60 0 135580000 3
to /boot/userconfig.txt instead of /boot/config.txt.
This has the advantage that these entries remain untouched when Volumio gets updated. In contrast /boot/config.txt gets overwritten on each Volumio update (further information) and you would have to edit it again.
Since max_usb_current=1 and hdmi_force_hotplug=1 are already part of the “standard” /boot/config.txt there is no need to add them to /boot/userconfig.txt.
But placing the above settings to config.txt or userconfig.txt is not the problem here. The reason you only see a grey screen with the mouse cursor is that the chromium browser is not running. Please check if it is installed at all and if so what version (chromium-browser -version)?