Welcome. The M-Lok version won't be as light as the Keymod....though the KM vs. MLok war seems to have been settled in MLok's favor, I'm not really fond of it on the XCR since it makes the handguard even thicker. I haven't mic'd the material thickness in the KM handguard on an L, but it looks to be quite a bit thicker than the BCM KM uppers I have and the breakage on KM was primarily due to the material being too thin.
I personally really like the FAST stocks...though I believe they could have been better implemented and still have room for improvement, but many here are fond of the ACR stocks adapted to the XCR via custom made adapters. You may want to look at that option too.
A 5900 SN is somewhere before mid 2012....probably made in early 2011 or so. JFYI.