I've used a multitude of rails across my builds. YHM, MI (gen2 SS and T series), Geisselle, Troy, Samson/Rainier, BCM, Noveske, etc...
12" is a good length, for me and my wingspan 10" is about minimum for how i like to hold the firearm. It's OK to go longer, but longer also entails more weight. Some like the look and some need the real estate on those long rails.
Most rails (all that i mentioned above) are free floated and handle weight fine....though i wouldn't recommend hanging a 25 lb dumbell off the front of it. The only place the contact the firearm is on the barrel nut so your gun will be theoretically more accurate than your current setup.
Keymod is intended as a license free standard for mounting accessories. The idea is that what you buy (grips, light mounts, rail sections) will work across any MFGs rail, as long as it's keymod. It's not the case with non keymod rails. MI panels that are not keymod will not fit a samson, centurion, etc...I haven't had the need to use panels due to heat on any of my keymod rails, but i don't dump mags very often either.
I'd think the Fortis is GTG but i don't have experience with it. BY FAR, my favorite rail i've used is the BCM KMR. Magnesium aluminum allow so the 10" rail only weights 5.5oz with barrel nut, 7.7 iirc on the 13". 3 of my 4 ARs wear the KMR. The one that doesn't is because BCM does not make a 7" for my 8" SBR...it wears a Noveske, and while the Noveske is good, it's not as good as the KMR.
If i was to go cheaper, i'd likely go with another MI. I used the Gen2 SS in the past. The SSK would be a good option as well.
The bottom is a 16" with 13" KMR. Top is 11.5" SBR with 10" KMR.
This one's a 14.5" with 10" KMR.