Gluing together personal projects is the foundation of the UNIX operating system. It's the point of any operating system really, but UNIX does it best. Docker and the microservice paradigm have also had a meteoric rise, probably because they promise to plug together clean slate personal projects.
One management technique I've seen companies use to grant employees the freedom to pursue personal projects, without it being seen as treachery, is to have a contract that says the company owns everything your mind produces, and then define quarterly expectations. That way you can sprint for a month doing what management wants, and spend the rest of the time inventing things like voicemail for fun.
I worked at a place that had explicit “20% time” to work on “whatever” and it (incredibly) turned into more micromanaged bullshit. Now I had to set separate goals and milestones for that project too, and report it in one-on-ones with my manager. I just dropped the whole thing because it was literally more work than doing my regular job!
Try saying, "I work on this project every fifth business day, so how about we discuss it at every fifth meeting?" Micromanagement is when a manager tells you how to do your work. Goals and milestones is a management task, so it's not an unreasonable ask. If they're asking you to write those yourself, then maybe they're wondering if you'd make a good manager. Imagine yourself as the 10x developer who needs to report to the director on what the other 9 developers are (1) doing and (2) intend to do.
Also, who cares if it's more work? Programming for me is not just a job but also a hobby. So if I'm working a job where the employment contracts states the company owns everything I do during the 168 hour work week, then my 20% time usually becomes 160% time (80% + 80%). You have to be willing to go to great lengths to keep people happy. Particularly the people who end up working on your project as part of their 80% time.
One management technique I've seen companies use to grant employees the freedom to pursue personal projects, without it being seen as treachery, is to have a contract that says the company owns everything your mind produces, and then define quarterly expectations. That way you can sprint for a month doing what management wants, and spend the rest of the time inventing things like voicemail for fun.