That's fair enough. It's unfortunate that adding fillets is a big design change in OpenSCAD while it's something you can easily add last-thing in other packages.

Because it's so programmable, most things can be made fairly easy if you think of them in time, by writing procedures, once. I'm interested, actually: I would have thought that long-time users would have developed some sort of tip or trick for this, recognising at the start the kinds of changes that might be difficult to make laters and planning them in at the start. Maybe internal rounded corners are just too much hassle any way you look at it.