As a tactical focused game I am surprised this is not there. I know the DMG has some optional rules for it, and that the game is limited to the SRD.
So homebrew a flanking rule. The DMG flanking position are already wacky for flanking a large monster so do something more sensible. Giving ADV on flanking is too powerful as well... Consider something less like a +2 to hit when flanking, and maybe only +1 if the attacker is also currently flanked.
This is an easy one to do without copyright infringement (do something different then the DMG says). As a tactical focused game you need this.
I'm a warior, I... war...