Welcome! Lot of ideas here I like. I've thought of exactly that switch arrangement - ON/OFF and mag slider for brightness.
Actually what I have in my hand right now is pretty close, just that the LED's setup is different - cooler tints, 3 LED's in parallel, 4 18650's in parallel, and running firmware that uses just one electronic switch, but has a smooth ramping ability, triple-click that blinks out the voltage level of the batteries (ex; 4.1v blinks out 4 blinks, then pauses, then 1 blink, does this continuously til you click).
I'm using this diffuser: http://www.fenix-store.com/fenix-aod-l-diffuser/, that fits the SupFire M6 perfectly. I bought mine on Amazon for $6. I'm really impressed with how well the diffuser fits the M6, and how well it does in producing a real lantern effect to light up a room or area.
Of course this light can do 5,000 lumens on max without the diffuser, but still, it can do anywhere from sub 1 lumen to 5,000 lumens on a smooth ramp of 150 levels in about 2.4 seconds (source code and manual are posted/published).
This "soup can" style light, like the SupFire M6 is great for very long runtimes on low lumens settings, with multi LED's and 4 cells in parallel. Lot of things you can do with them in customizing. Two projects going on now are a 4X LED customized high amp group buy here:
https://budgetlightforum.com/t/-/39069 (4X Neutral White LED's in the 5000K range or so)
and a pretty cool lantern mod for camping of the same base light here:
https://budgetlightforum.com/t/-/38215