New to diesels, power and shifting questions

I just bought my first diesel truck, 2016 Silverado 2500hd LML diesel. Crew cab 4x4 LTZ, 44,000 miles. After driving for a couple thousand miles I definitely notice some shortcomings and things I have tried to improve, but still need improvement.

1st, seems like the truck is constantly holding power back? From a dead stop and flooring it, there is a huge lack of power, I'm sure they do this to protect other parts of the truck. I would like to have all the power there and I can make the decision on how to use it or not.

2nd, the transmission is very sloppy and unintelligent. 1st-2nd shifts good, 2nd-3rd shifts good, but once I hit 3rd, even if my foot is to the floor it immediately shifts to 4th, then 5th deleting all momentum and pulling power dead in its tracks. This is very frustrating, why would the tranny shift with my foot to the floor knowing I need that power?

3rd is down shifts, or lack thereof... It takes what feels like 4-5 seconds to downshift and feels like it only goes from 6th to 5th with almost no additional passing or climbing power...

I added a stealth performance module and notice pretty much 0 improvement in "dead pedal" feel or low to mid range power (I wrote them and didn't get any real help). I also installed a Dans TCM with custom tuning and don't notice any improvement in how the transmission acts.

Is there anything available to help the tranny hold all gears better when I need the power instead of completely falling flat, including downshifting to 3rd or 4th. Any better power improvements to help the wide open throttle restriction or dead pedal when flooring from a dead stop. I am not opposed to buying a fully built transmission if whoever builds it can add some sense to this thing...

Thanks for any help,
-Mike