The problem here in DavinciResolve is forcing it to update its cache... But as any CS student knows:

There are only two hard things in Computer Science:

• Cache invalidation
• Naming things
• Off by one error

attributed to Phil Karlton (and Leon Bambrick for the third one)

The way to signal DR to invalidate its clip cache is using the Media Pool right click operation "Replace Selected Clip..." . So here's my workflow in DR 18.6 (tried in macOS and Ubuntu):

1. Externally edit a clip (modifying image or sound)
2. Save the new version alongside the original one, with a slight name modification
3. Back in DR: Find In Media Pool + Replace Selected Clip...
4. Chose the new modified clip
5. Tadaa! Refresh is done!
6. If another external modification is needed, save under the original name And alternate back and forth between the two names for each refresh