Rather than LP, it's better to have a BD made into a custom nsp
Vanilla Brilliant Diamond will never receive another official update from Nintendo, it is permanently stuck on version 1.3.0 Luminescent Platinum, however, gets updated by its developers fairly often
If you make the Vanilla BD the custom NSP, it becomes a "build once and forget it" solution You will never have to repack it again Meanwhile, LP gets to stay on the standard Title ID, meaning you can just drag and drop the normal LP update files into your atmosphere/contents/ folder like you always have, without ever needing to use PC tools again
The Setup
You will use a PC tool like NSC_Builder or Switch Army Knife (SAK) for this
Take your clean base dump of Brilliant Diamond and the official 1.3.0 update file
Use the tool to merge the base game and the update, and select the option to change or patch the Title ID to something custom (e.g., changing 0100000011D90000 to 0100000011D90001)
Repack it as an NSP and install it via Tinfoil or DBI. This will pop up as a brand-new icon on your Switch. This is now your dedicated Vanilla BD