Libra was released in September 2019. I didn't study if there is a regularity in realizes of Kobo devices and it is not simple to imagine the future but anyway I don't think that there would be an imminent refresh of the Libra H2O.
The owners of it in this forum generally are satisfied with it so it is never a bad time to buy a Libra
.
The new Kobo device that will be soon released seems to be a new Forma (this is the prevalent opinion in the forum).
About the 7" screen: I have a Kobo Aura One 7.8" screen and there is a very great difference with the other two 6" devices I own. With a 7" screen of course the difference won't be the same but I think that You will note it.