I've been playing around with blowback Colt 9mm setups since around 2003. Using them for SMG competitions and was always disappointed in how choppy they run. Best I could do was run the LRM suppressed ported barrel setup and an old Olympic Pneumatic buffer which eventually started to leak and was useless.
Always wanted an MP5 as it is in my opinion the gold standard when it comes to smoothness in a closed bolt 9mm SMG.
Now that the 9mm Guard came out, I've been playing with it and so far I'm pretty happy with it.
I got just the 8" barrel and BCG combo and stripped Guard lower. As mentioned, I'm using a Tubbs flat wire 556 spring, A5 tube and Blitzkrieg/Kynshot hydraulic buffer. Very smooth. Very close to a full auto MP5K PDW.
Note that the Guard carrier accepts the same weight as a Colt 9mm bolt and obviously is machined to trip an auto sear.
Once you get the carrier modified, I don't think you would have any issues running the Guard with your LL.
Unlike a Colt pattern blowback 9mm, I have never experienced any trigger slap with the Guard in any buffer/spring combination I have tried.