There's nothing wrong with flexwatt, we've all been using it for years. Two of my racks and my bator are flexwatt. I only have one rack, my newest rack, that is THG heat tape. I personally still call THG flexwatt, to me it's just the updated versionAlthough, if you have the option to use THG I would use that since it doesn't get as out of control when you run it wide open (or your thermostat breaks). It still gets way too hot for bps though, when I ran it wide open it got to 130+ degrees, but took a bit of time. My flexwatt hit 150 degrees within minutes before I unplugged it. I know it's designed with reptiles in mind, but I don't think it's "way safer" than flexwatt based on my own testing. IMO flexwatt and THG are equally dangerous if something goes wrong.
I would also use 6 inch with a 28qt tub.
I use Ranco, Helix, and Vivarium.