I have been working at my current company for almost a year now. Things are going well and I'm happy working there. I am now on a temporary contract (1 year) but this is ending soon and I expect they'll hire me for indefinite time.
Now, besides work, I have this side project that has some potential. Several parties have already expressed their interest and while this is of course no guarantee, I would like to try and get it to the market (and of course hope it 'takes off' :) ) But, I don't have a lot of time to work on this, next to my fulltime job and volunteering.
I was thinking about the possibility to work parttime (80% - 4 days) to finish my project in the day that I am off, but I do not want to kill my 'career'. Ideally, of course, I can make enough money with my project later and I quit my job altogether but this is not something I forsee for the direct future (like, next year or so).
What are the downsides to going from fulltime to parttime job for this kind of reason? Does the reason even matter? I don't want to become a manager or something, but I also don't want to be the 'part-time employee' that gets left out of every opportunity.
For context, I'm a software developer living in The Netherlands