I've searched through the documentation looking for guidance on how to best modify Official templates. I wasn't sure if I should be copying the template, and modifying the copy, or if I should just modify the original. And then, what happens when an updated version is released? How do I maintain my modifications but also update the template to the most recent version?

Below are excepts of existing documentation talking about the need to tune templates, and how to upgrade them, but none of them answer my above questions.

Section 12 - performance tuning
general advice:
tune 'Update interval' for all items. Keeping a small update interval may be good for nice graphs, however, this may overload Zabbix
tune parameters for default templates

Installation -> Template Changes
Note that upgrading to the latest Zabbix version will not automatically upgrade the templates used. It is suggested to modify the templates in existing installations by:

Downloading the latest templates from the Zabbix Git repository;

Then, while in Configuration → Templates you can import them manually into Zabbix. If templates with the same names already exist, the Delete missing options should be checked when importing to achieve a clean import. This way the old items that are no longer in the updated template will be removed (note that it will mean losing history of these old items).

Quick Start -> 6 New Template
LINKING PRE-DEFINED TEMPLATES TO HOSTS
As you may have noticed, Zabbix comes with a set of predefined templates for various OS, devices and applications. To get started with monitoring very quickly, you may link the appropriate one of them to a host, but beware that these templates need to be fine-tuned for your environment. Some checks may not be needed, and polling intervals may be way too frequent.

More information about templates is available.

Configuration -> Templates out of the box -> Overview
(tells how to update templates)

Configuration -> Templates out of the box -> Zabbix agent template operation
3. Adjust the values of mandatory macros as needed.

Git repo - Template readme.md
(no mention of best practice)