Hrmm, based on the amount of corrosion on the hardware my bet is your barrel adjuster is probably frozen. Additionally the cable routing to the anchor assembly seems off. Start with penetrating oil on the threads of the barrel adjuster. You might need some pliers for extra leverage to free up the barrel adjuster.
Also with that much corrosion I would unseat the shift housing along the cable to check for corrosion (which will create resistance) and at least lubricate them with some light silicon oil (chain lube will work)
In the highest gear check for excess cable slack. Correct your cable routing (should be straight through under the plate, look for a groove as a guide) and if there is slack pull out the excess while the pinch bolt is loose.
Then start the process of indexing the gears.
Replacing the derailleur would be an option depending on how stuck the barrel adjuster is but chances are good for breaking it free (drilling, retaping and replacing the adjuster is probably too much effort for that model derailleur). If indexing keeps being a problem check the housing and cables for corrosion, consider replacements. They are cheap and will make shifting much smoother.
I am using a Hifi Walker H2. It is a reasonable price, has bluetooth and no capacitive touch inputs. The UI is not great but the device is physically solid and works for me.
I pre-ordered a Tangara on their crowd funding. I am hopeful about the opensource hardware and software. Only reservation is capacitive touch scroll wheel.