BCM 11.5" BFH (Or a Centurion 11.5") is solid, stick with that.
Handguard - the DD 9.0 Lite is good if you want rails on all four (lighter than the TROY bravo equivalent).
The TROY/VTAC Extreme is among the lightest handguard options, but...
The Noveske NSR 9.0 is going to be even lighter than that. I'd go VTAC if you don't want the keymod setup (my 9.0 VTAC-X SBR is very light).
Rainier did have a really good price on the AAC Mini4 22 cal can (I think it was just under 7 bills), so that might be the can to look at value wise if you're after a light, functional dual stamper. That setup would be the same overall length as a Mk18 (10.5" Bbl) with a full size can, but it would run a bit lighter for sure. It'll run just fine with an A2FH until you get the Form 4 Stamp in for the can (in pistol or SBR form) and be a really light rifle.
Next details are what stock, grip, and weaponlight configuration you're going to want to run (pretty much assuming Aimpoint T-1 on LT/ADM/DD lower 1/3 mount)?
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"Being able to do the basics, on demand, takes practice. " - Sinister