I like this topic, because I too am familiar with many plant names I've only seen in print. Being a logophile, I have researched some pronunciations out of curiosity but others I just garble in my head.
One linguistic fact that helps a lot is that many plant names come from Greek root words (that carried into Latin). The Greek "ch" is pronounced like a "k" โ as in chamomile, chaos, chemistry, etc. Hence kal-an-ko-ee. But that is even disputed between American and British English experts, so I say, pick the one you like.
When I first saw Agastache, my initial reaction was ag-ah-stash. Then the penny dropped about the ch, and I wobbled between ag-ah-stak and ag-ah-STAK-ee. Then I watched a video in which the owner of an online nursery said a-GAH-sta-kee. Okay then!
One I really like (Greek roots again) is Caesalpinia. Think Caesar, the emperor. That initial "C" works as an "S" and makes it fun to say: sez-al-PIN-ee-a.
I have a feeling this thread won't have mass appeal, but thanks for the opportunity to geek out!