I will go against the flow of reason here and say: you might regret it... if you don't do it.
Life is too short. In no time you will find yourself much older, forever working on other people's crap projects you don't care. It will mean a whole world of difference if you made something for yourself. Even if it failed, who cares, you at least tried.
Whether you tell details to your current employer it depends on the business type. It is expected of developers to have small side projects to stay in touch with ne tech. Reasonable employer might still be happy as you will perfect yourself to current technologies which you cannot implement in present workspace, you will be motivated as you still need that paycheck. In big corporation however it might be frown upon so you might leave details out. Coming from small caring company to big bank I just cannot comprehend how there is no sense of greater benefit but instead prevails inefficient, petty, shortsighted quickest result mindset.
I am working in the bank sector and situation of Covid has actually increased the demand for developers because it has show that lot of processes (surprise, surprise) can be done and should be done through the intranet/Internet. If things go south on your current job, you should find next one without that much trouble.
And think about the future. You will get old, and developer hierarchy is usually flat. It's hard to get into lower management, and it might not be your cup of tea. You can follow trends when young, but sooner or later you will not be able to keep up. Ceramist, carpenter and plummer will have their name proved across the decades of hard work, their phone number will be ringing and they can send their apprentices to make work and collect money.. you however, won't have any. Having small trickle from side job might be great thing.