Many way to skin a cat. All the approaches above could work.
Personally, I would not tap the rearview as that circuit is sized and designed for the loads already present. You're assuming there's enough margin on that circuit to support a dashcam, and while very possible, it's just as possible with some higher power dashcams might draw enough to blow the original circuit.
There's two interior fuse boxes under the dash at the footwell behind the cover with unused circuits that an add-an-tap can pull from.
I'm pretty sure the fuse box under the driver footwell are all switched on with the ignition in the on position. The passenger footwell is constant on. Choose the source that is appropriate for your use case. Then run the wire up along the door trim and headliner. You'll need to remove A-piller cover (10mm bolts under the grab handle), to run the wire under the curtain airbag.
View attachment 3458055