No difference in softproof image but major differences in historgram

You should see a difference. But you don't. Because Lightroom color management is utter broken and useless for the purpose of seeing what prints will look like.

The correct answer is: import your image into Photoshop and use its color management and proofing tools, which actually mostly work. You can load an ICC profile for, say, a metal print, and the image you see onscreen will (often drastically) change in appearance, as it should. Lightroom just doesn't do this and offers no cogent explanation for how to achieve this result. Photoshop works properly; Lightroom doesn't work at all.

Note: even Photoshop suffers when it comes to brightness. Monitors are often set much too bright, and although prints previewed using a specific ICC profile in Photoshop will have their colors displayed properly for that profile, their brightness may be way off. The only reasonable fix for this is: Select your ICC profile, make adjustments in Photoshop if needed to color-correct for that profile, then spend the money to have a small print made. It'll likely come back too dark. Using that print as a reference, adjust your monitor brightness until there is a reasonable match to the print. Note, too, that brightness matching like this is itself somewhat fraught; the actual lighting in the environment where the print will be displayed needs to be taken into account, too. But this approach will get you quite close in most cases, and overcome the disappointment when you receive a print that seems shockingly dark compared to what you saw onscreen.

This is how soft proofing works. Ideally you should NOT see any difference. This means that all the colors are inside the gamut of both your monitor and the icc profile. So that you don't see a change is completely expected. Oftentimes the biggest changes are observed when you also enable "simulate paper and ink" but that option is just meant to give you an impression of the impact of the paper color and the ink black density. With it off in general the difference for soft proof should be minor except if you're soft proofing for a not too good printer. If you're soft proofing for a different standard color space like soft proof for adobeRGB it is extremely rare to be able to see any difference. Sometimes proofing for sRGB you might see a difference if you are on a wide gamut display.

The histogram that will be shown in the histogram display when proofing is based on the icc profile instead of based on the Lightroom color space for the histogram which is a special colorspace with prophotoRGB primaries and a sRGB tonecurve. These profiles usually have completely different tone curves than sRGB and absolutely have different primaries. So the histogram should change but the appearance of the image should not change. When you see large changes, it usually just means that the image has colors outside of the soft proof profile and you can choose to correct for that (but don't have to if the difference is minor or not objectionable).

In a first step please try resetting the preferences of Lightroom Classic:   https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

Not necessarily. You are looking at the soft proof on a monitor, so there are two color conversions: First the conversion to show the soft proof, but then these colors will be converted to your monitor and that is where you can lose the visible differences.

As an example: Let's say the image is ProPhotoRGB, you soft proof against AdobeRGB but your monitor is sRGB. Because sRGB is the smallest color space of the three, you will not be able to see on your monitor what soft proofing from ProPhotoRGB to AdobeRGB does.