Rough ride

The alcohol content issue is common on GMs that have flex fuel engines. The most common cure is to use a scan tool to take a sample of the fuel, test it for alcohol content using a simple test and then resetting the alcohol content back to the baseline. The easier method is to just reset it and let it relearn the content after the reset. Having it wrong can cause a myriad of drivability complaints as the system assumes the content is correct and will adjust the various items like timing, fuel mix and cylinder deactivation to match the supposed running conditions the high alcohol mix needs. However, if that alcohol isn't actually there it will run rich, smell and run rough and act sluggish. The colder weather, and winter blended fuels may have masked it a bit. The reset takes care of that.

For the suspension I suggest getting a second opinion at another shop. The OE parts are very expensive, and you can get aftermarket units cheaper, but they are still close to $300.00 each for the rears and $400.00 each for the fronts. They could be leaking or damaged but unless you are actually feeling that or the truck wallows around while you are driving, they may be fine.
I had a transmission replaced in a vehicle under warranty and the shop tech told me that "both front struts are gone because they leaked out" except what he saw was actually the undercoating oil I have used on vehicles for years.
In your case you could easily slip under the truck and look at the shocks and struts, if the parts around them are all dry but they are oily, maybe they are leaking, wipe them off and see if it returns. However unless you think it rides bad or seems to bounce or wallow I would suspect the diagnosis. There are ways to remove the system, but it takes a dealer to remove the option code from the system to keep the active suspension failures from showing on the dash if you remove them.

The ethanol issue happens even if you only use straight no booze gas. It has to do with how the fuel sensor detects the alcohol and how it measures the content. Nobody has so far pinned down a definite answer because it seems to occur regardless of the actual alcohol content.
Your description of the ride doesn't sound like a shock or strut issue but it just having a truck suspension design under a large heavy vehicle. If I remember that system can be adjusted for different ride quality. However, you might want to look at https://www.shockwarehouse.com/news/gm-suv-suspension-decoder.cfm
to get a better idea of what you have and other options.

Okay, you have the Z95 option which is the "fancy" version of Magnaride. That is the fully electronic controlled suspension option. It is the same thing they have on the up-level Cadillac's. It uses a controller to activate the fluid inside each shock, no valving involved. That is why you have the connector on the shocks and a similar one will be on the struts.
You can use the base level parts to replace them and use the bypass kit to shut the system off.

Once the parts are replaced, a quick front end alignment should be all that's needed. I would probably unplug the connectors, shoot some dielectric grease into each one and put a wrap on them, just in case you change your mind or sell it and that person wants to go back. Plus it would help prevent anything happening if the connector touched something else.

In OEM condition the Magnaride is very nice but if you have the OE level scan tool you can also play with the calibration to make it softer or stiffer. The Cadillac version gets the soft ride, the others get more truck like settings. GM could have easily made it user adjustable, but they didn't do anything until the 2021 versions, and even then, it's minimal.
Owning a scan tool that can adjust it makes a difference.

Okay, if yours have an air line it must have the automatic leveling and Magnaride as well. That is about the most expensive system GM offered at the time. The aftermarket doesn't really offer much other than ways to eliminate it, which is what the parts you have should do.
The dealer will use your parts, likely because they cannot get the OE parts in any reasonable time and want your business.
The only reason for a scan tool would be to reprogram the suspension control module after you removed the OE magnaride parts. That is what the piece you bought from Xineering should do.

That would be if you replaced the suspension with OE parts. You mentioned you were using nonstock parts that didn't use the air system. If you intend to keep the auto leveling system intact and use OE parts, the scan tool would be used to set the initial height if you replaced the control module or a height sensor. I don't think I've ever done that when changing out just the rear shocks themselves.

Z95 has Magnaride at both ends plus air at the rear for the auto leveling. If you only have an air line to the rear you might want to check it closer, it almost sounds like someone already changed some parts. Your dash sticker shows the Z95 option which is 4-wheel Magnaride.
Arnott used to make some that fit but I haven't looked in a while.
These look to be Z95 compatible, but you need to see what parts of the system are still there before buying them.
https://www.amazon.com/Genuin-Absorber-Kit-Part-23487280/dp/B0128HZBVK

The issue I would look at first is if you still have the entire 95 system on the truck. You want to verify if you have the control module and wiring before you buy the parts. I'm wondering if you have the Magnaride parts up front and only the load leveling in the rear, which could give you a real strange ride that might explain the problem.

They look like they would work. I never heard of them myself. I would go hunting on your truck first though and verify that the rest of the system is there first.