How can Cumulative Updates to SQL Server 2019 be Slipstreamed into a Single Installer? I'd like to take the latest version of SQL Server (SQL Standard, 15.0.2000.5), and apply the latest CU to it so that I have a single installer, thus reducing the size of the resulting package. The custom installer that I'm developing packages the SQL Server installer, and thus reducing the size by several hundred MB is important.

In previous versions of SQL Server, it was possible to create a single merged media output folder using the /PCUSOURCE and /CUSOURCE parameters, however this seems to have been removed in SQL Server 2014 (See "Slipstream Functionality" here). The alternative provided is to use the /UpdateSource flag, however this approach only allows for the simultaneous application of patches at the time of installation (by bundling the CU media with the original installation media). Further, the /ACTION=PrepareImage facilitates a SysPrep installation, not a slipstreamed update to the install media.

So, is there a way to update the installation media of SQL Server Standard 2019 with CU patches such that there's a single (reduced-size) installation media bundle?
You can't do this, unfortunately.
As I wrote in my blog post here, you must:

Download and extract the installation media to a directory;
Add the patches to an updates subdirectory (yep, all of them).

As you've discovered, you use the /UpdateEnabled and /UpdateSource switches when it comes to doing the installation.
The one small mercy is that CUs are cumulative, so you only need the latest one in the updates subdirectory.