Tilda can definitely be a stand-alone name. Some people might assume it’s short for Matilda and ask about it, but if you’re introducing her as Tilda, then she will be called Tilda. I don’t think it anyone would actually call her Matilda.
Tilda Marigold is a nice pairing. Tilda very much strikes me as strong and sturdy, along the lines of Zelda and Frida. It’s also unexpected, although you might get a lot of Tilda Swinton comments as that’s the main association in Australia.
I still prefer Kaia Summer for you. I think the cool, down to earth vibes fit Kasper and your overall style a bit better. Also being a consistent favourite of yours gives it the edge over the new addition of Tilda. Either way, I’ll be interested to hear your announcement when baby arrives!