I just can't get elements to display correctly

Just in case someone reads, the plugin in the conflict was Asset CleanUp: Page Speed Booster.

So if I follow you correctly I should create this header banner in Blocks editor and then merge with Elements header?

So if I follow you correctly I should create this header banner in Blocks editor and then merge with Elements?

Exactly :)

I did the same for Target without the merge:
https://gpsites.co/target/

That won’t work unfortunately.

What you could do is to create the whole page hero part with GB as the first section in the content, then create a page hero to merge only:
https://docs.generatepress.com/article/transparent-header-and-navigation/

I've tried creating a new header hero but I'm so lost what to do. The documentation doesn't help since there's no button example. I can easily place a text/image on the header element but I have no clue how to merge a button on.

You don't actually add the text or button in the page hero.

What I'm suggesting is to create the whole thing (image, text and buttons) using GenerateBlocks. This is what I've done for Target:
https://gpsites.co/target/

Then the header element is simply to merge the content (GB header) with the navigation:
https://docs.generatepress.com/article/transparent-header-and-navigation/