As long as the bootloader isn't locked, porting mainline Linux shouldn't be as bad as with E-ink devices. Here's a Ubuntu Touch port to another Helios G99 device [1]; I really hope the RLCD doesn't require host driver support.
You need USB-C DisplayPort (DP Alt Mode). Unfortunately it's not a software limitation oftentimes, most of the times it's a hardware design limitation. I guess Onyx figured that they can have two different product lines and milk both, as opposed to what they used to have, which was an HDMI port on the higher end models.
Best would be DP alt mode. However software to send video over regular USB data lines is fairly common and reportedly works okay so long as you have drivers (which generally are not supported to the next Windows release) and are not trying something too heavy - not good enough for games but often good enough for the office type work the OP is asking for.
[1]: https://devices.ubuntu-touch.io/device/vidofnir/