GE produced the CT-7-9C3 and the 9D2 both with over 15% more take-off power and 5% more cruise power with optimised metallurgy, air flow and electronics which also reduced fuel consumption. Obviously both were not considered enough of an upgrade for the SAAB 340, whether cost or operational savings.
I still think that you are underestimating the cost of an upgrade spread over a small fleet. Technically everything you write is doable, maybe without too many challenges, but even the simplest thing these days seems to cost 3x what you think it should cost in even your most fevered imagination.
Cost is factored per unit, you go to SAAB (Boeing, ATR, Airbus or whoever) and they basically tell you how much it would cost per unit over your fleet. SAAB already has the zero time program in place for the SAAB MPA, which could easily be converted to airliner specs just swap out the Naval Patrol interior for a passenger cabin, new cockpit, new wiring and electronics etc. The issue right now is not how much it costs or whether it can be done, but for how long this technology will be valid. No point spending $5mil per aircraft to have a smick fleet of dinosaur burners when electric is just around the corner. That is if the crystal ball said fossil fuels the only way for the next 20 years, you'd see more upgrades, right now everyone knows there's a technology change in the wind, which could render all the old tech obsolete and uncompetetive. Regionals have to budget on 20+ years for major fleet changes, as opposed to larger airlines that bank on 10 years for a Boeing.