Considering moving from L-mount to Nikon Z

First, a little premise.

I have been shooting with a Sigma fp L for about four years now. I love that little camera so much: it's small and compact with a great industrial design, it has incredible dynamic range, great noise performance, excellent colours, and the 60MP sensor has so much detail you can crop down all you want.

Buuuuut... it has no IBIS or a mechanical shutter, and it doesn't use a stacked sensor so indoor photography always has banding unless you shoot at 1/50s and hope the line frequency is close enough to 50Hz, and of course shooting handheld at 1/50s introduces motion no matter how steady you are because it's such a high resolution sensor.

So as much as I love it, I think it's time to upgrade. Unfortunately, L mount only has a handful of advanced cameras, and for a real upgrade I would have to get the Lumix S1Rii or the Leica Sl3-P (the SL3 has a few issues that don't make it as compelling as the P, even though it has a lower resolution sensor, plus the price difference is unjustifiable). The Lumix would be the best option for the price, but ignoring the price, then the SL3-P is the better camera and I'll probably get that.

Now, with the price of the SL3-P in mind (I can get it for 5000€) I have been looking at the Nikon system and specifically the Z8. Switching to it would probably end up costing me about the same amount as getting the SL3-P once you account for the money "lost" selling the lenses I have and the Sigma fp L and getting new Nikon lenses to replace them. The Z8 would be a big upgrade, not in image quality (I have looked at many raw files from both and can get the same results no matter the camera to the point that I cannot distinguish between the two) but in functionality: it has a better autofocus, higher burst rate, better EVF, etc. In short: same photos, better camera.

Of course, staying with L mount lets me keep the Sigma fp L (it literally fits in a pocket it's so small) which is a big plus, though I could get a Nikon Zr in the future to have a more compact option for travel (25MP are plenty for vacations and while it lacks a mechanical shutter its readout speed is high enough to avoid issues).

Is there anyone with both systems that can give me a proper comparison?

How is the lens selection for Z mount? L mount has a LOT of different manufactures with all kinds of prices and quality levels, whereas for Nikon it doesn't look like there is the same range? At least not as many options at different prices.

Sorry for the long post and premise but I am a bit stumped. Getting a new camera is a big investment and I don't wanna risk getting into a cul-de-sac like I kind of am with the L mount now, and I am in doubt whether it's worth it or not.