As some other comments mention, billboards can be great for more impulse type businesses. You see it, you need it.
The main use outside of that is being a piece of brand repetition. You can get your name in front of people through multiple channels to drive it in. This only makes sense if you’re actually using a multi-channel ad strategy.
Plus, a ton of billboards are horribly designed. They put way too much on there. Consider the average driver seeing them isn’t highly focused on them, can’t immediately look up anything, and that billboards are far away. Keep it simple using only your name, briefly what you do, and where to find you. Anything more than a few words in overdoing it.
100k can go 100 ways. There’s some questions to answer here.
What do you want this to be? Is this meant to be a side hustle, or are you looking to grow something to become your primary thing?
How much time do you actually have? Are we talking 10 hours a week, or 40? Be realistic about this number.
Are you okay losing money for a while til you get to scale, or do you want immediate returns? What is your risk tolerance?
What technical skills do you have? If you’re looking to do a service, it’ll depend on what you can actually provide.
Do you want to be online, brick-and-mortar, make house calls? Where are you wanting to provide from?
If you’re not comfortable answering those, I’d stick to what you know and parking the extra in a safe investment.