Just as a general experience from myself I never had good success on any kind of converter adapters regarding DP, HDMI, DVI or VGA when using with ARM boards. No idea what the cause is. Maybe general lack in drivers to mitigate such configurations...
If somehow possible try to connect native.
Did you leave the microSD in after move? Booting directly from USB is not possible afaik but you can have the boot loader on sd which then boots from USB.
If the system was running flawless before installing OMV and does not after I suggest to blame them 😄.
What armbian-config more or less does only is downloading the omv install script and execute it.
https://github.com/armbian/config/blob/eebfc58c140a34d6e28db5ecbc5faba6b8432ca4/debian-software#L559 ff.