If you want a gallium driver, it is.

I am not sure if recompiling Mesa is necessary. If you are going to recompile, you don't need radeonsi and r600. They are both radeon GPUs. The i915 is intel GPU. Also, I do not use Wayland, so I left that out of "--with-platform".

Before recompiling, you can try executing the nvidia run script and see if it works. If it has a problem with mesa, it will complain.

I think the main requirement is to omit DRM and nouveau in the kernel config and not compile the xorg nouveau driver. However, for most distros, the nouveau and proprietary drivers are able to coexist. You can prevent a conflict by blacklisting nouveau and preventing it from loading by setting
in /etc/default/grub.

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As a followup, even though I specified without nouveau and r600 when I compiled Mesa, the makefile went ahead and built them anyway. I can see the libraries in /usr/lib. That said, everything works fine. So I don't think it is necessary to recompile Mesa. You could also compile the Xorg nouveau driver without any side effects. The user can decide which driver gets loaded via kernel parameters.

I would build the gallium swrast and svga at along side my actual drivers. I know that both of those can act as pass-thru to other backend drivers because there are multiple paths for instructions to come through.

This turned out to be an interesting thread. I found out that the nouveau and nvidia proprietary drivers can coexist in BLFS just as in other distros.

If you wish to use nouveau, you have to compile Mesa with drm and nouveau support, which the makefile wants to do anyway. Then you compile the kernel with DRM and nouveau as modules. Also, compile the Xorg nouveau driver using the BLFS book.
Startx should then work using the nouveau driver.

Now, if you wish to install the nvidia proprietary driver, you must boot into a blacklisted mode using the parameters I specified above, and execute the nvidia run script. If there are is no compilation error, startx should work.

If you wish to revert to nouveau, you have to delete /etc/X11/xorg.conf and un-blacklist nouveau in /etc/default/grub.

A good indicator of all is well is
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IMHO it is better to use the proprietary driver, assuming it is available for your GPU. No firmware blobs needed and much easier to install.