I remember back about 25 years ago I worked for a GC and he was hired to tear down and rebuild decks/balconies on an apartment complex. He hired a two-man subcontractor crew to do the work but decided to pay them by the hour (no idea why). As I recall, the two men would do two decks over a span of three weeks. The GC decided their labor costs were killing him and renegotiated to pay them a flat rate per deck. Their production rate shifted from two decks in three weeks to three decks in two weeks.
I understand where are you are coming from in wanting to make your pay structure more efficient.
However, I am concerned about the issues you may run into as far as Fair Labor is concerned and minimum wage. What happens if a "bad" tech rushes through jobs in one week and spends half of the next week on callbacks, without pay? Or what happens between August and November, when the workload is light?
I think you are going to have to pay an hourly rate no matter what, and then use the "hours per job" system as a basis for paying out bonuses.
Also, you mentioned the hard workers are currently being penalized by having more work added. With the "per task" pay scale, I assume they will see bigger paychecks and now get rewarded for their hard work, which is a good thing. But your "not hard workers" will at the same time see a drop in pay, possibly? What happens after that? Do they turn in their notice and go to a competitor who is still paying by the hour (and possibly offering a higher wage right now to deal with the labor shortage)?