Mirrorless lenses, especially the not astronomically expensive ones, seem to generally be slower lenses. DSLRs are most amazing when full frame, and lenses that are f2.8 or faster.
Really? They have a 20 1.8s (which I have used the crap out of and it beats the 20mm1.8G) , 24 1.8. 28 2.8, 24-70 2.8, 14-24 2.8, 35 1.8, 40 2, 50 1.8 and a 50 1.2, 85 1.8, 70-200 2.8 -- dude, if you can't find a faster lens that suits your needs in that range you're really picky.
And Nikon is behind on lenses. Other manufacturers got a better start out the gate. The selection will grow with time.
Personally, I do damned well with the 20mm 1.8s and the 24-70. Fast, beautiful lenses. I have the 24-200 which IS a slow lens, but for certain things I shoot it'll be daylight and the all in one is worth it so I never change lenses. I won't rave about that one though, it's just a good lens but not great.
But I can very easily find fast lenses for when I'm working at night or shooting a band with marginal stage lighting.
Also, and this is likely just as true of the latest DSLRs as well, I'm really happy with the Zs higher ISO performance. I can buy a couple of stops there that I couldn't on the Nikons from 10 years ago.
But, and I do NOT think this is true of the DSLRs, the extra VR boost from the Z helps for static shots in darker circumstances. A lot. An extra stop or two a lot.
I don't use the VR as much as I could because I shoot moving people, but for just scenes I've been able to get sharp shots in candle light and way into blue hour at ridiculously slow shutter speeds by leaning up against a tree and shooting handheld. The VR has gotten that good.
For the OP, the question is just how much he is sticking with his current lenses. If he's using them, the Z lens lineup is moot. But if fast native lenses are the thing holding him back, that's moot.