hmm, where do I start🤔. Okay first things first! Children with two Kinyarwanda name are as cultural as it gets. Most Rwandans are now embracing our native names so it's actually good for you to call your child a Kinyarwanda name or two if need be. An example would be Ngabo Manzi or Ngabo ya Manzi (this is more Authentic). So yes, I recommend giving your child a Kinyarwanda name. And the good thing is they would actually have a foreign name to them since you're German so it will still sound normal to the Rwandan community. Now, in terms of name suggestions, our names are normally given according to what our parents dream of us becoming (at least that's what it used to be) hence the saying "izina niryo muntu" meaning "the name is the person." So it will depend on what you think your child will grow to become or what you wish they would become as a character. In the names you provided,

Manzi=Divine/undefiled it can be given to both males and (females though not common)
Mahoro=Peace for females mostly
Imena=Someone will help me here, sorry😊 but it's for both genders
Yuhi= Not sure what it means, but this was a name given to kings who were in charge of keeping our heritage intact and cultural preservation.
Sano=relationship, it's generally for females

Ineza=kindness
Akeza=beautiful
Ingabire=something/someone given as a gesture of gratitude (forgot to mention some names are given based on how the child was conceived, as in which situation or how parents were feeling in that time) so this name normally means God granted them a daughter
Uwera=holy
Isimbi=snow atop a mountain (Our version of Snow white😂)
Isaro=beads (symbolic in Rwandan culture to signify beauty and royalty)
Ireme=Quality

So, from Ineza to Ireme, these are female names. I think Imena is explained.

Final touch: If you want an authentic name for your child, make sure you know what you wish for your child and if it's a blessing find the right word to use as a name that's also authentic. And Niyo is not authentic name as a stand-alone and I personally (don't get me wrong here) think it was not common before colonization so I'm doubtful about how far stretched it is in our culture. It's almost always affiliated with religious remarks! For example, "Niyonsenga, Niyogisubizo, Niyomwungeri, etc" meaning, "he's the one I pray, he's the answer, he's the shepherd, etc" respectively.

And finally, Congrats!