Non-PHP tips wanted for custom default settings?

I have recently done by first two multisite installs. I know CSS but do not want to modify PHP at this time.

All my sub-sites start out with the same Theme and needing the same Theme adjustment settings. But each time I “create new site,” I must start fresh with out-of-box defaults. I must re-do navigation columns, widget configurations, etc. This goes beyond custom CSS.

I believe I can use “site backup.” My theme also has a “settings backup.” Install my standard Theme to any site -> set desired custom default settings -> Backup the site or settings -> Create new site -> Import backup from previous site. No experience yet, but I believe this will work.

Method #2: create Child Theme. This also could do the job. But I do not want to do any PHP modifications, and my default settings go beyond CSS. Child Themes make PHP updates “simpler,” but still not simple enough for me, I think.

Also, I do not understand PHP. I suspect that all the settings are saved in MySQL database–the PHP files are not “written” with these setting adjustments…? If so, I assume the following will NOT work…?

-> set desired custom default settings -> then copy all affected PHP files into Child Theme -> Child Theme will have custom default settings, no need to “import settings” -> Probably not, I think…?

Therefore, I will soon begin experimenting with “site backup” and “settings backup” imports between sites. But if by chance Child Themes might improve this process (without needing PHP modifications)… or if anyone has any tips about transferring “site” and “setting” backups between sites… or any other ideas or methods… this would be appreciated. Thank you.