RED FLAG!!!
Any particular reason why you decided to override the template defaults, and go with an unsupported, experimental, OSX-guests-only option? Please change it back to the default PIIX3. Do not change the defaults unless you know what you're getting into.
<Display controller="VBoxSVGA" VRAMSize="128" accelerate2DVideo="true"/>
3D acceleration is not enabled, you might want to enable that.
<Controllers>
<Controller name="OHCI" type="OHCI"/>
<Controller name="EHCI" type="EHCI"/>
</Controllers>
Since you already have the ExtPack installed, you could enable xHCI (USB3).
<Image uuid="{2302bbfa-89bd-4a6a-9b25-4510e23e600b}" location="/home/steve/Downloads/VBoxGuestAdditions_6.0.14.iso"/>
...
<AttachedDevice passthrough="false" type="DVD" hotpluggable="false" port="1" device="0">
<Image uuid="{2302bbfa-89bd-4a6a-9b25-4510e23e600b}"/>
</AttachedDevice>
Get into the habit of removing the GAs ISO once you're done installing the GAs. And you should not be downloading the stand-alone GAs, unless specifically instructed to do so. There's a major possibility that your main VirtualBox program and the GAs won't match.
Finally, can you please be specific about what KB fails to install? I'm really not too crazy about generic statements like "an update failed to get installed", that update should have a name, no?