You don't need tubeless for going up a tire size and to a lower pressure. I've happily run 25, 28, 30, 32, 35, and 38 mm tires on my touring bike with tubes. If you're going from a small tire (26) to a large one, it's probably a good idea to buy a tube made for those tire sizes . . . but that's just a few bucks.
My experience is that large tires running at lower pressure will flat less often than high pressure and narrower tires - regardless of whether they're running with tubes or tubeless. If you're getting flats because of punctures, there are several extremely robust types of tires you can use (Schwalbe marathon plus, Continental gatorskins, Continental tour ride, Specialized Armadillos, etc.) These tires are designed to be much more resistant to sharp objects than typical bike rubber, and will last much longer (at the expense of weight and sometimes ride quality).