Upgrading the transmission - where to begin?

I’m going to do the first fluid and filter change in a few weeks and figured this post would get me started in the right direction.

2015 Tahoe LT 5.3 L83?? See VIN below
6 speed 6L90e?? See pics below
53,000 miles
No current concerns
CARFAX doesn’t indicate any transmission service to date.
Besides the severe maintenance schedule (would someone mind posting a copy of that please and thank you), what might be the first few most cost-effective things to upgrade on the transmission?
2+ quart pan?
Lower temp sensor?
How many quarts does this thing hold and what type - Dextron 6?
Also, is my exhaust pipe supposed to be flattened on the bottom? (see pic)

Also, is my exhaust pipe supposed to be flattened on the bottom?

Yes, the exhaust is designed that way.

The transmission holds just over 12 quarts in total, six quarts are exchanged when dropping the pan and replacing the filter.

If yours were mine, the first thing I would do with the filter and fluid change is to put in that lower temperature thermostat or "flip the pill". Being in Clearwater, you don't have to worry about the transmission being too cold!

I see a lot of rust or corrossion underneath, is that from boat ramps or Yankee winters? I would address that too with some kind of oil and cavity wax.

Transmission is a 6L80. A gallon of GM Dexron VI Full Synthetic is under this part number: 10-9244 or 88865618.

You can download a digital copy of your owner's manual in PDF form on Chevrolet's website. Our site does not allow such large files to be posted.

Ok, so I will start with the fluid and filter change but what is flipping the pill? LOL I will look in other threads for the how-to.
How hot is transmission fluid rated to get up to and still not break down too fast?

The rust and corrosion is from her first 3 years in Yankeetown and I plan on coating it with PB rust prevention.

Do I also drain the torque converter? I did this on my 2000 Expo and was able to get about 10? of the 12ish quarts out

CHECK on the transmission version and exhaust pipe, thanks

I took a screenshot of the basic recommended schedule picture someone posted in another thread - is the severe maintenance schedule located in the owners manual? I am on the Storage section 4-1 currently (why are these things so fun to read?!!)

Should be located towards the back of the manual under maintenance. No need to read the entire manual unless you just want to. In the digital manual, you can just click certain areas in the glossary to go where you want.

Sounds weird but I’m enjoying the read!
Found it - man, I thought it would be along the lines of what I read members do in here like changing transmission fluid every 30-50k, etc.
Is AC Delco an acceptable brand for GM OEM replacement? I found their trans fluid for $33/gal

Pulled boat in sunny, 75 degree FL weather and she went up to 195.

Any suggestions for this first transmission filter and fluid change at 54k miles? Dipstick shows her pretty brown.
I think I will suck out from the dipstick, and then drain upper trans line exiting radiator and drop the pan for the rest (flipping the pill at this time too). That should remove about 8 quarts correct?
Then I will add new fluid and driver her for 1,000 miles and then repeat the process all over again but this time replacing the filter with a new one.

Thoughts?!?!

UPDATE:
So I went a head and changed the transmission fluid and filter last weekend.
I read and reread a couple threads on this forum in order to come up with a plan. It helped SO much!!
I began with using an oil extractor pump and pulled 4 quarts out through the dipstick. I then went under the truck and pulled the transmission thermostat housing and ‘flipped the pill.’
I then replaced the thermostat housing and dropped the pan and changed the filter. I chose to leave the original filter seal in place and reuse it. I was able to suck out another quart from the bottom of the pan.
Call me lucky but I didn’t have to mess with the exhaust as most mentioned. It definitely took some wiggling and shifting back and forth for about 5 mins but then she released.
I’ll post a pic of the inside of the pan and extracted fluid.
Cleaned her up, used a new gasket and new filter and put 6 quarts of fresh Dextron VI down the dipstick.
Then I moved to the upper transmission line off of the radiator. It was crazy how fast and how much fluid came out! I chose to use a 3/8” OD hose to fit inside the radiators fitting and had to keep firm pressure to keep it inside while the pump was pumping fluid. I added another two quarts and ran it again until it sputtered.
All in all, I retrieved 11 quarts of dirty fluid with 54k miles on it, 9 of which was very nasty. It is such a great feeling to get the old out and bring in the new.
As far as the transmission temperature since flipping the pill (Florida temps in March are high 70’s low 80’s) I was running 190-200 consistently. Now I’m mostly around 145 and she’s been up to 156. I towed the boat and she got up to 180. I’ll be curious to see how she does in the heat of summertime.

I plan on flushing just the fluid in 25k miles and fluid and filter in another 50k.

Thanks for everyone’s support and contributions!!