Read More not displaying after writing Excerpt.

Did you take a look at the second block of code here?
https://docs.generatepress.com/article/activating-read-custom-excerpt/

It's quite different than what have above.

And when you are using the excerpt metabox, WordPress assumes you want to displays the entire content in there so the trim won't work.

I've switched to the second block you mentioned, still quite long

The block of code is for adding buttons when using custom excerpt, which is your original question Custom excerpt = button disappears
https://generatepress.com/forums/topic/read-more-not-displaying-after-writing-excerpt/page/2/#post-666336

And looks like it's working now so I would appreciate if you actually try my suggestion next time instead of repeatedly saying you are already using the code:
https://generatepress.com/forums/topic/read-more-not-displaying-after-writing-excerpt/page/2/#post-666317
https://generatepress.com/forums/topic/read-more-not-displaying-after-writing-excerpt/page/2/#post-666328
https://generatepress.com/forums/topic/read-more-not-displaying-after-writing-excerpt/page/2/#post-666336

As for why the trim isn't working, I've explained in the first sentence here:
https://generatepress.com/forums/topic/read-more-not-displaying-after-writing-excerpt/page/2/#post-666314

and again in last sentence here:
https://generatepress.com/forums/topic/read-more-not-displaying-after-writing-excerpt/page/3/#post-666343

It doesn't make sense to use the custom excerpt (which doesn't show up in content) then trim part of it off so no one will ever see it. Then you might as well write an unfinished sentence.

I will close the topic now.