So ...

If the first two methods won’t work with your theme, then you can create your own custom excerpts by ...

3. Using a plugin

It's called Advanced Excerpt. Here it is:

https://wordpress.org/plugins/advanced-excerpt/

It's a really small plugin (85 KB), but let's see the usual speed tests ...

Here are my results before installing the plugin ...

... and the result after installing and activating the plugin:

No more comments needed ...

So, let's see how it works ...