The rarest achievements showcase doesn't let you edit it the same as the regular showcase sadly.
It will only show the ones that are determined to be the "actual rarest" ones rather than the rare ones you worked for that you'd want to show.

One option would be to purchase an additional normal achievement showcase in the points shop so you can add that above your other achievement showcase, and put the rare ones you want to show off in that one instead of using the rare showcase.

But what you could also do is mark the game as "private".
Hiding the game as private makes it so the achievements and the game don't display on your profile to friends and others, only showing to yourself.
To private the game you go to your library, right click the game, go to manage, and then enable "mark as private".

Sadly this happens with many games especially those whereby users earn achievements on DLC that other users don't purchase. You can then be tanked with lots of 0.1% achievements all from the same content.