Congrats on the build! Most buck drivers can operate over a broad range of input voltage and will deliver whatever voltage the emitter needs automatically. There should a good number of buck drivers that can handle automotive voltages and deliver 6 amps to your emitter.
The HX-1175b is pretty robust. I don't know who has the best price these days. Here is the first link I found. If you don't want the 5 modes (L, M, H, Strobe, SOS), you can jumper the PWM pin to the MCU's VCC pin for single mode. I don't recall the stock current that it is designed to deliver. I think it's about 7.5 amps.
There are probably some even lower cost options, if you don't mind a rectangular shaped driver (which it appears you would have no problem using in your build). I'm just not aware of which ones will work for you at this moment as I have not shopped for buck drivers in some time now.
Best wishes on your driver quest.
EDIT: Here are some builds using the HX-1175b delivering various amps MT-G2 or multiple 3v emitters in series:
https://budgetlightforum.com/t/-/26076#node-30670
https://budgetlightforum.com/t/-/28798
https://budgetlightforum.com/t/-/34779
Here is a thread on the HX-1175b:
https://budgetlightforum.com/t/-/26978
EDIT2: Sorry for all the edits. Kept thinking of things I forgot to say.