First off, is it weird to give the third child two middle names when the first two only got one?
What are you Berries’ thoughts on [name_f]Imogen[/name_f] [name_f]Katherine[/name_f] [name_f]Johanna[/name_f]? The last name sounds like “mantis”. The reason for those middle names is that they’re variants of all four great grandmothers’, both grandmothers’, plus my sister’s and my name as well.
I think the three names are nice together. I think it’s ok to use 2 middles even though your others have just 1, although [name_f]Imogen[/name_f] [name_f]Katherine[/name_f] on its own does feel more balanced with sibs. If it’s important to you to honor as many family members as possible with this one, I say go for it!
My sister and I each have one middle name and our younger brother has two. As far as I know it isn’t weird and hasn’t been a problem. I’ve always suspected that our parents knew they didn’t want to have more than three children and since he was the last one and the only boy, they figured it was “now or never” as far as honouring both grandfathers was concerned.
[name_f]Imogen[/name_f] [name_f]Katherine[/name_f] [name_f]Johanna[/name_f] “Mantis” works really well! (Impressive to be able to reference so many names all at once!) If I had to nitpick, I don’t love how close the “an” sounds are in [name_f]Johanna[/name_f] and “Mantis”, but I think that’s pretty minor, and better than having the soft G/J sounds in [name_f]Imogen[/name_f] [name_f]Johanna[/name_f] close together (which if you flipped [name_f]Katherine[/name_f] and [name_f]Johanna[/name_f] they would be). Well done. 
Thanks, y’all! This will most likely be our last baby too, so that is why I’m trying to get them both in there. Maybe I’ll end up with twins. lol Then I can give each of them one of those middles.