I flash hazards usually, and on the rare occasion where someone flashes hazards for me i “respond” with double highbeam. Same when i let someone in, building a gap and then giving double highbeam to indicate space and clearance. Grew up with it (father being a trucker), can’t shake it off.
In regards to more “less illegal” means.. I’m not 100% on the legality, but there’s always this.
Or this, if you’re more affluent.