There are quite a few threads on making a negative positive…
Here’s a quick selection:
- Negative Conversion
- Automatic negative conversion
- DXO Photo Lab works great with color negative scans
- Invert film negativ in DPL 3
- https://forum.dxo.com/t/converting-old-negatives-in-op/1777
- https://forum.dxo.com/t/invert-function-any-chance-of-getting-one/431
Again, check out https://www.negativelabpro.com for more hints,
also about capturing with VueScan… NLP uses techniques in order to make very good conversions easily. Tone curves can get very refined, depending on what I set.
Here, the three curves look similar because the negative was easy to convert.
Difficult negatives will produce curves that look quite different.
Manualy adjusting so many points on the tonecurves is not practical. Manual adjustments can be limited to something as shown by @rrblint above, still providing decent results.