Here's the exact process:
I start in Scene, making sure the top-level Spatial (which everything else is a child of) is selected. Then, I double-clicking on the sprite I want to import (all .glb). Then click on New Inherited, make sure it's set to RigidBody in the Import tab and, if necessary, set it to RigidBody and re-import. Then, back in Scene, I make sure it's RigidBody (I discovered that sometimes, no matter how you import it, it still wants to be a static body) and change its type if it isn't. I click on the Clone (mesh), and in the Mesh tab, I add a single convex collision sibling.
Then I save the scene.
After that, I click on floor, then the chain link and instance the above new scene into the floor scene.
Most of the time, that works just fine. But for a few, well, they like to move around on their own. Now, in the previous (scrapped after I gave up with this, certain that I'd done something horribly wrong, one of those was the spikessmall ... and this time, it's spikeslarge. Could it be something in the sprites themselves? It does only seem to happen with specific ones.
I think I mentioned this, but these are all from the Kenney Platform Kit, using the GLTF format.