Just use a money cheat and buy all the DLC weapons. Most of them are upgrades from the default weapons and are available from the start.
I thought this too since they are on the OG versions, but on PC, weapons that were DLC on consoles are now integrated into the game in a way that makes you need to unlock them like any other wep.
...unless i'm missing something, but i don't think so.
it's the same with cars, all DLC cars for original console versions were available for free in your garage, now they're not and have to be gotten like any other vehicle.
I like it this way, so much more stuff to unlock, it's one of the reasons why another play through makes sense and is fun, because you get nearly 2x as much content as the original game.
edit: speaking of new content, fuck first person view when driving the Duke of Death, it's like you're looking through a letterbox