Just like you, I struggled to optimize my first WordPress website to please Google. So, I realized that if you don’t know what themes score without optimizing with plugins.
Over the time, I came to know that themes from WordPress team, are the best performing themes. When it comes to page load time, accessibility, coding best practices. ( It does not mean that other themes don’t meet these requirements. ).
It’s just that WordPress-themes are easy to find, and everyone gets the latest one installed by default.
I create a child theme of the latest theme by WordPress team and customize it to meet the desired UX/UI.
There are two things, I do after creating the child theme…
I minify JS/CSS/HTML
Keep compressing the images
That’s all I do…