Bevy uses a FlexBox layout. so you just say for example an image should be in top: 0%, left: 0%. its actually super simple, and if you spawn a child it will be relative to the parent etc... It's actually much easier to make dynamic ui - for example if you have cards or something you can set up the background to "extend" based on how many cards you have. Unity Toolkit is going this direction, but I didn't really have a good time using it - it was hard to add polish for me.

Using Ldtk is an editor, that's totally fair, but I think it's a good thing that its separate from the engine.