Not a lawyer but I would probably just keep all side projects on the down low, don't draw attention by asking for a copyright assignment exemption, don't develop it at work or even log in via a company pc or phone. Don't mention it to co-workers. Keep your name off of it on the public side. Keep it anonymous. I expect you could even use a 'pen name' if you wanted to, if you need to be visible, do podcasts, etc.

Most of your side projects aren't going to amount to anything anyway but if you keep it anonymous the chances of your company approaching you about it after you leave are very small. Even if it turns in to a success.