Avoiding rebuilds (e.g. of mariadb, the entire Rust chain, etc.)?

[Matthew Brooks]

Is there any way to avoid rebuilding stuff like mariadb, the entire Rust chain, 
etc. unless one of those packages *actually* changes? It seems like every few 
days every single package needs to rebuild for some reason, including many 
packages that spend unbelievably long times running tests that will never 
actually be of use to me, so I'm usually only able to update every couple of 
weeks since so much constantly needs to be rebuilt and everything takes so long.

It seems to have gotten worse over time as well, as more and more base packages 
pull in extremely computation-hungry dependency chains. At this very moment, 
for example, I'm waiting for Rust & co. to compile simply to delete old system 
generations from the bootloader.

I've got substitutes turned off (primarily because I like the idea of every 
package being "home built" as it were), but I can't imagine that even the 
official substitiute servers would be able to keep up with the constant 
rebuilds my system seems to want unless I've got something horribly 
misconfigured somewhere.

The definition for the guix service in my config is:

(modify-services
 %base-services
 (guix-service-type
  config =>
  (guix-configuration
   (inherit config)
   (use-substitutes? #f)
   (authorized-keys (list
         (local-file "./laptop-key.pub")
         (local-file "./tablet-key.pub")
         ))
   (substitute-urls '())
   (extra-options '("--gc-keep-derivations=yes" "--gc-keep-outputs=yes" 
"--no-substitutes"))
   ))
 )

I've virtually zero experience with Scheme or Guile, so maybe there's something 
obvious I'm missing, or some obvious way to change the behavior, but I've never 
been able to find anything about it.