Öffi (https://f-droid.org/packages/de.schildbach.oeffi/) claims that it offers Sydney, but I’ve only used it in Europe.
Where I’ve used Öffi, it’s at least as good as the official app, sometimes better.

My Pixel is taking short video snippets from which you can then choose the best frame, and so far no other app than GP is able to handle those (as far as I know at least).
Can Immich work with those?