It’s great to hear you are doing well with your CSS success.
I probably have a different site requirement than yours. As I go along, I’m making changes on the style.css and I have a custom PHP functions file. Also, I edited other PHP files as well like header.php, footer.php to suit my liking and these two files exist in the parent theme, I copied them to my child before making the change.
Just imagine, if your theme is updated to the recent version, any changes you made to these theme files will be overwritten. If you do not want your efforts gone to waste, the way to avoid that is to have a child theme.
When you troubleshoot an error, isolation would be easier because in the child theme, you will have less files to look at where you made a mistake. Coding, especially PHP is unforgiving, a missing comma or bracket can wreak havoc on your site.
Trust me, I was grateful I created a child theme before I made a mess to my own blog. Thanks to a WordPress moderator who reminded me to refrain from editing the parent theme’s files directly.
This could be irrelevant to your question but I recommend that you do a regular backup of your site. Good luck!