The denoiser built into DAZ is a bit too zealous from what I've found, and DAZ settings are designed to apply the denoiser many times over multiple iterations, which can lead to an odd semi-painterly sort of look. If that's not what you're going for, I've found the
Intel denoiser to be quite good.
As for DAZ not using your GPU:
Scene Optimizer can also help minimize your scene's VRAM footprint. Also, make sure you have the right driver version for Iray - Daz 4.14 requires at least 451.48, but when I was on Daz 4.12 the newer drivers
wouldn't work, and I had to use (I think) the 430.86 driver. Also, Nvidia Iray won't work on AMD cards at all.
Also, there are ways to set up your scene to reduce noise, so you get a better image faster. It's good to have LOTS of light, for one thing. For night scenes, you can overdo the lighting, then reduce it with an image editor. It also helps for light sources to be physically large; the realistic setup, where a lightbulb has a tiny super-bright filament mesh shining through a glass surface, is very bad for noise in my experience. An emissive, opaque bulb material is better, or (better yet) have lots of your light come from large invisible area lights (look at "Light Geometry" under the light's properties, and play around with the settings).
In one scene I had a ceiling fan model that with frosted-glass bulb covers, which turned out to be a
horrible source of noise - the scene got much better after I made the bulbs (rather than the filament) an emissive opaque material, and covered the whole thing up with a 180-degree disc-shaped spotlight just underneath the model, to provide most of the light for the room.