Did I do anything legally or ethically wrong?
Transfer of knowledge, as long it doesn't include trade secrets or actual IP, is legally OK.
Use of your time in side gigs you may wish to double-check:
Eight hours a week is not much - are you really containing your efforts into that time? How do support issues with your "side gig" work get resolved without impacting time on the main job?
Are you burning yourself out and reducing your efficiency at the other job? This is not necessarily an ethical consideration, because an employer should not have control over what you do in your spare time. However, if you get no rest at weekends, or too many late nights, then your performance could well slip at the larger job.
An employer might still fire you with cause if your performance slips, or you juggle your time too much for their comfort - it doesn't matter whether it is because you spend your time partying or on a side gig. Likewise your side gig employer might appreciate the cheap product you have made for them, but might become frustrated at low level of support you are able to provide.
Check your contracts for clauses that cover conflicts of interest in general. It would be ethically and maybe also legally wrong to be moonlighting when you have signed a written agreement to not do so. Clauses in employment contracts requiring your employer to effectively be your sole employer are relatively common, due to concerns on performance and on leaks of IP (even if you fully intend not to leak IP, your employer only has your say-so that you are not, they cannot audit the work for the other employer).
Also, do bear in mind, that although you feel within your rights to act in a certain way, and it could even be legally backed up, your employer may decide otherwise. Perhaps they are still concerned about possible loss of their IP, and would fire you or even take legal action against you. Even if you would win such a case, you may not wish to go through the process or have it affect your main career, for the sake of whatever money you can make on a 20% part time role.
Bad scenarios could be more likely if you have kept the side gig secret from your main employer. You can do a lot towards clearing whether your employer thinks this idea is ethical and legal by asking them about it. You will also get your chance to frame the possible conflict of interest as something that you will handle professionally, before it gets discovered any other way.
It is probably more important to you that your main employer has a positive view of the ethical and legal status of your side gig, than the Q&A on this site.