Yeah, after writing that comment, I was thinking, if I do promote it, that means there's a certain expectation that I'll integrate or implement functionality that others want. At that point, it becomes less of an egoistic thing. And I'll be doing more communication and whatnot, therefore less programming.

Maybe that's the puzzle piece that OP is missing? If you don't promote it, you have practically no extra work compared to developing it under a proprietary license. In fact, it often reduces the workload, if you can just post it publicly without having to secure the repo.
And you don't incur costs from giving it away either. So, if you make sure to only put in the work that you want to put in in the first place, you have no disadvantage from publishing it with an open-source license.