Hi!
I finaly got lvgl 8.3 to work with my esp32 in espressif ide and squareline! 
I found a working project in a github issue. Thanks to icodk to change the esp32 drivers so it works with lvgl 8!
You can use the driver component from this project and then clone lvgl 8.3 to your project as component. It’s important to clone release 8.3. If you don’t specify this while cloning, you will clone the master branch. This doesn’t work because they are doing the development of lvgl 9 in there.
After that, you need to call some functions to do the initialisation of lvgl in your main c file. This is described in the porting guide of lvgl or you can use the lv_port_esp32 example as reference for this.
Lastly, you can put the generated squareline files in the main folder (or another one). After you included the c files in the CMakeLists.txt file and the ui.h in the main c file, you can call the ui that you designed with squareline!
I hope this might help somebody. If you have any questions or need a more detailed explanation, let me know!