[Vedor]

For me, a great middle ground is building an engine similar to fantasy consoles (PICO-8, TIC-80, etc.)

This type of project is small enough to plan all the elements before you start (which helps to stick to the design and therefore to avoid feature creep). Yet, it is still complex enough to provide entertainment and challenge.

This approach also has the advantage – at least it is an advantage in the case of side projects – that you can see the results of your work quite quickly.

I'm working on such a project myself, and it is a great experience. Although to tell the truth, it is more of a "game creation kit" than an engine.