Hi
VP is excellent at producing video slidshows from images and I don't exactly understand where your problem lies as I don't see a deterioration (or hardly any) in the output of a slideshow video. Here is an example. It uses a basic tif image 4480 x 2520 (16:9) of 11meg.
This is the ORIGINAL image. On the right is an enlarged part of this image.

Loaded to VP I took screendumps of the PREVIEW PANE both UNZOOMED and ZOOMED to match the enlarged area above.
In this example under Tools/Options/General I had the resolution of the preview set at 512 x288 (16:9) which is the lowest preview resolution.

It's obvious that the zoomed in area of the preview image is not too clear.
By re-setting the preview resolution to the highest value 1920 x 1080 16:9 the preview image is much better. Particularly when zoomed in as seen here.. The unzoomed frame looks hardly changed.

For the best editing resolution it is therefore better to use the higher setting as I mentioned in my post above. (also c-major and Borate) The problem here is that VP has to render the previews etc at the higher value and as a consequence takes longer to do so or even pauses frequently with generating preview messages. It depends on the speed and capacity of your PC.
In this test I outputted the zoom sequence as an MPEG4 at the highest resolution of 3840 x 2160 4K
The encoder settings were for the highest quality (1.0) The effect employed in VP was a simple ZOOM into the frame as above, to produce a similar enlargement to the original image. Here are screenshots of frames from the exported MPEG4 VIDEO. On the left the unzoomed section and on the right the zoomed section.
MPEG4 Frames

Comparing the zoomed images of the MPEG4 and the original tif (even if they are converted to jpegs and have less pixels than your own) I can't see a particular loss in definition. (If anything the MPEG4 is better.)
This would imply that the screened MPEG4 slideshow would look little different from images shown on screen in a more traditional way;and that's been my experience.
Try the setting suggested and see if you see any difference.
Nat