Hi Florian,

since I am not very familiar with programming, I used the code exactly as you showed here.
The only differences are:

1) block name "core/gallery" instead of "soliloquy/soliloquywp"
2) "Displaying the rest of the content" code is being printed BEFORE the "Display the slider block separate from the content" code.
3) $blocks = parse_blocks( get_the_content() ); is declared before "Displaying the rest of content"

You can see here the result in my WIP website:
https://www.creativenergy.it/portfolio/sports-mania/

When I print with , Simple Lightbox plugin could apply the lightbox to the Gutenberg generated gallery. After that I print the filtered content, and then the splitted core/gallery. In this case the Simple Lightbox plugin can't apply the lightbox to it.

Here is the php for this template:

https://gist.github.com/Ksasa78/005e2aa5852bf7d29acb72f0cf02b33d

Hope you can figure what's going on!