My degoogled android died a year ago and it happened before I bought that watch. In the meantime waiting for my librem 5 I bought an iphone for basic phone use.
I would guess that you can install the apk on a degoogled android when you are able to get the apk separately. I did it with other apps before and if I am not mistaken it was a desktop app called racoon which helps you to extract apks from the play-store.
You could try it. The viita-apps works in demo-mode, when you don’t have a watch yet.