How cool would it be if they offered a simplified furniture CAD program, where you could choose what material you need how many shelves of what size, etc and it would generate the needed program that would drill the required holes, and would tell you what size of screws and pins and dowels you need to pick up.
I definitely would use something like that, instead of trying to cut preexisting furniture to size.
Look at the very popular Lack table, it's made from cardboard hexagons with four pieces of wood-like thing in the corners to hold the legs. You can't make that yourself for cheap, and just the material cost in eg. wood, would be more expensive than the full table in ikea.
Otherwise, there are many youtube channels specializing in woodworking where you can learn all the basics to design your own furniture, but it won't be cheap.
As cheap as Ikea was, being in London in my 20s just as Freecycle started, meant we ended up with a lot 'vintage' furniture for free or next to nothing - some we still have now.
Looking into ideas, I was a little bit disappointed to not find community website of people sharing ideas, experiences and results around such hacking.
Ever since then, I avoid sharing pictures of IKEA furniture. Like Mattel, suing barbie blogs of children. If I did not design it, I won't share a picture of it. Even building architects will sometimes sue if you photograph the house.
Now that you have published this online, police will visit you to remove the Billy!
Perhaps I am too pessimistic with German laws.
PS: It looks much better this way.
Now I will either get older recycled furniture for quality or pieces from IKEA for cost.