Hi @MichelyWeb
You have to load the splide.js in the block so that it uses it in the back-end and front-end. The initialization code you can put also in the splide.js at the bottom. Since you can only load one js file in the block.
That should work. Been a while that i did it that way. I now am using Alpine.js to initialize the slider and that works great.
Guido