Been following this thread for a while now and thought I would share my findings so far.
I purchased mine Jan 2020 and it now has 22k miles on the odometer. Zero issues so far and I'm still in shock with the fuel mileage. I've never been able to keep anything stock and I'm constantly searching for improvements to every facet of life. This truck is no different. Though, I knew it would be difficult given the EPA's record of tyranny and hypocrisy in regards to diesel vehicles. That's a rant for another day...
So, I first did the DIY turbo resonator delete while I waited on my backordered S&B intake to ship. Not a noticeable difference on any level, but more peace of mind that the air path was now round and smooth for optimal performance. This was done at 15k miles and there was already a decent amount of oil in the hoses and resonator. Not sure if you can tell in the pictures. But, nothing that can be done about that (though I was hoping I could put a catch can on it).
Second was the long awaited S&B intake (I've been messaging S&B since last summer). All I can say is WOW! In both performance & sound & in the PIA department for install! Most ridiculous "bolt on" I think I've ever encountered. I'm not a technician but very much mechanically minded and capable and it took me about an hour and a half! But hearing the glorious sound omitted after the install made it all worth it
Most recent (this past weekend) I finally received and installed a Fleece Fuel Filter Upgrade (https://dmaxstore.com/products/?route=product/product&path=1066_1879_1890&product_id=7327). Our fuel filter PN and housing are the exact same as a 2020-2021 L5P so I figured I'd give it a shot and see if it worked, and it did! Only difference in the Fleece instructions and the LM2 is the fuel line fittings. Unlike the easy to remove blue tabbed fittings, ours have the "Ford White Clip" as my local NAPA guy called it. And, I honestly don't think they are meant to be removed... Long story short I broke one in removing the stock housing so I had to find a replacement. GM does not sell the clips (furthers my thought they aren't removeable) but I was able to find an exact match from NAPA (PN 730-7380). Since it wasn't listed to fit the LM2, I figured there would be some hiccups but apart from the clips, it was a piece of cake and bolts up and fits perfectly! Highly, highly recommend this upgrade. Actually, going to do my dad's LM2 next weekend.
Next will be some form of tuner. Trying to hold out for the Derringer because I want the iDash as well for monitoring. An Air Lift bag setup (I don't tow often but when I do it's usually 5-8k lbs) and I'm leveled so the squat gets real... Then maybe some water/meth injection. Plan is to keep this truck for a long time so performance (optimal efficiency really) and longevity are my goals with any mods I do to the truck.