Separate child themes for each child site

I have a directory WordPress Multisite, the core theme library is Astra, the mother site and a number of sub-sites have obvious differences in appearance and functionality, and I intend to give each site with a separate sub-theme. I've encountered the following confusions, and would like to ask friends with experience:

• AstraNeeds to be activated across the web to keep the parent theme unified and up to date

• Each site loads only its own child theme for easy customization of CSS, JS, and PHP.

What is everyone's best advice?

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