If you work in games or want to work in games, writing a game engine is a very valuable experience. Probably the best thing you can work on for personal growth.
That said, I think you get substantially more value out of the experience if you've already gotten your feet wet making games with a commercial engine.
Now obviously QJ2D is not going to replace Godot or even LÖVE2D or TIC-80. It's a couple of days' worth of work, less than 200 lines of code; anyone could have done it. And nobody else is going to use it unless they have Greek letters on their keyboard. But not only did I learn a lot, it's also a substantial improvement for this kind of thing over the raw browser. I think I'll probably rewrite it differently for the next project, though...
I don't want to "work in games", but of course I enjoy writing games. I mean that's all computers are good for, really!
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† not really games