What other mods/versions do you have? Did you try it with just the delayed loading mod enabled?
Any error messages output to the console when it's failing? There's a few potential causes.
1: Godot just breaks when more than a certain number of objects added to the ScrollContainer. So maybe at around 4000 things worked, but at 6000 things just break? If so I'd expect to see some errors in the console.
2. My mod uses a bit of a hack to render the initial viewed items. Without the delay, the rendering engine hasn't setup the size/location of everything yet and so it thinks everything is visible and the delayed loading ends up loading everything. I thought the way I did it the actual delay didn't matter as it's block on that compositing, but maybe I was wrong
3. Something I'm not thinking of, but console output might help identify.
Hmm ok so that could be either #1 or #2. I'll try and take a crack at my logic for determining which images to load to something that doesn't depend upon the compositing so if that has issues, it still only loads the necessary ones, and i'll test it out with 0.5.20a. Probably won't get to it till the weekend as next few days are busy.
I'll also probably take a crack at a really efficient design that uses a smaller number of button objects and re-uses them. Dealt with some similar designs for web pages, though doing the asynchronous loading in Godot will be a lot more painful than it is in javascript.