Check the SEO space, and see how many people are running ads on the main search terms, how hard is it to rank for the the main keywords. Check if forums exists, and discount coupons for similar products in marketplaces, check for similar products on product hunt and all the other product websites like Appsumo, Capterra, etc.
Look for blogs, Youtube channels, Facebook groups, subreddits. Join the communities and post there, ask questions. Interact one on one with at least 15 or 20 people and ask them about their needs.
You can do all this in a couple of weeks in the evening, without even writing a single line of code, and validate your idea.
Regarding the legal part, unless you are directly competing with your employer, they couldn't care less.
And even then, a lot of companies are created by people that learned the industry by working for other companies.
Your boss does not care about your side project, as long as you get the job done. But for you to feel better, don't put your name on it until you have quit your job, it's the internet you can still be anonymous.
No company is going to waste their time and expensive legal resources gratuitously suing you for a side project, unless it's something blatant that directly impacts their bottom line.
I wouldn't tell your boss about your side-project either. He will likely just see it as a sign of lack of dedication, that your priorities are elsewhere, and a tell sign that you will leave soon anyway.
So if they are on the fence on keeping you, that might trigger a decision.