- shitty enterprise software, on one of the worst code bases I've ever seen
- Managers are very hands-off here, not many meetings either
I was in a similar spot. You can get away with a ton of stuff in an environment like this, but as you've probably guessed, it will quickly bore you or the code base will invoke tears. Bonus if they let you work from home, and they probably would if you come up with a decent reason.
Check your IP agreement, because someone doing due diligence on your Android/SaaS business will find that out if you ever plan to exit with your new companies.
I'd be interested if you could 're-launch' your products with a clean version control history to sneak out from underneath an IP agreement since your current employer likely doesn't have any interest or any code from you regarding those unless you've told them. That would be a lawyer question though.
Ethically, you should still keep enough working knowledge so that you don't fall into incompetence, and realize you may need to do occasional crunch-time which should get 100% of your attention.
In my not-so-humble opinion, it's up to the business to assess your skills and make use of them, so by only handing you work which you can easily accomplish, you are fulfilling your responsibilities to your employer. This ultimately hurts you more than them because it ends up being low-quality stuff for your resume and you have to do more stretching of the truth. The business is already stretching the truth likely by promoting the shitty code base as a good product.
Naturally, they won't be happy to find out you've been taking advantage of them, so I wouldn't ever bring up your Android/SaaS stuff or your free time around anyone.