You are correct that only certain plans allow for full HD video in a Zoom call. And you are correct that you have to request the feature. There are a few flags on the account backend that Zoom has to turn on/off to make it happen, even if you have the 1080 options in your account settings.

Regarding video vs screenshare, yes screen share will always be alittle bit sharper, because all sources in Zoom are a balance of frame rate and resolution, with camera sources leaning more towards frame rate, which makes sense because it's a camera, and shared screen leaning more towards resolution. If you use the Zoom for vMix plugin and connect that way, it likely will put the vMix video into Zoom at the same sort of quality you're seeing with the virtual camera, but it's an option worth checking.

If the quality is as bad as you're describing, that's a different story. You can check the resolution from another machine, by pinning that virtual camera to your other computer's meeting window, and going into the Statistics part of the Zoom menus, and seeing what actual resolution is coming through. Be aware that a lot of times even if HD is truly enabled for the meeting and account, there still has to be a "demand" for HD from somewhere for it to kick in. Someone needs to have the camera window of your vMix virtual camera at a large enough scale to demand it be 720p or 1080p. I have taken the step in the past to dedicate a machine to pin a participant just to force that larger view and HD quality.

If you only need HD quality for single events and not long term, I believe Zoom Events temporary licenses come with that feature. I think they've gone to tiers with their resolutions, so a Pro account can do 720p, but not 1080p. And a Business or Enterprise account can do 1080p, but I'm not 100% on that. I will say though in my many years of experience with Zoom and event/production, that the difference between 720p and 1080p in Zoom is pretty small, especially when it's a camera source.

To answer your other questions, Camera sources will never be as sharp as screen share sources, but can be pretty close at 720 or 1080.
If you're sharing text, then yes...screen share will always be the sharpest looking option. If you don't like the idea of PPT or Keynote etc in use, consider a Stream deck with Companion running and controlling the computer feeding the slides in. It makes things very easy.