Baseboard is topped with a piece of cap molding, which is small and bends easily to conform to variations in the wall.Installing Baseboards - How to Install Baseboard Molding - Carpentry, Woodworking, Finish & Trim.
DIY: How To Build Up Or Layer Crown Moulding - this post shows how to space your mouldings and paint the wall between them - the mouldings appear to be wider, and more expensive than they are. The Home Depot.and don't forget the paint color❤️
Add a strip of trim a bit above already existing baseboards, paint between, and you get faux thick baseboards! Other good ideas here as well. 31 Easy DIY Upgrades That Will Make Your Home Look More Expensive