I admin a big craft group on FB, and have been doing it for 10 years+.
Over that time I've seen trends come and go, and watched lots of people build up really successful crafts businesses from scratch.
Selling candles? Yes, it can be very successful - but to do it legally, there's a whole heap of compliance and regulatory stuff you need to adhere to. Even if you only sell one handmade candle. Health and safety regulations means that craft side hustles aren't anywhere near as easy as they used to be - not if you want to do it legally anyway.
It doesn't matter if you're just selling your hobby - as soon as money changes hands you have to comply with legal requirements. And lots of those are more onerous than you might imagine.
For example, anything with a "play value" has to be safe for children. That applies whether or not it's a toy. If it looks like something a child might enjoy playing with, then it has to be safe for children. Going back to the subject of candles - you can't make candles that look like food unless it passes the bite test for children, and wouldn't be toxic if ingested.
When I say health and safety rules are stringent, I'm really not exaggerating.
The worst thing that people do is take up a hobby because they think it will make them money, and not because they enjoy it. And then they try and sell the first pieces they make because it's just about the £££ and not acquiring a real skill. And inevitably, the quality is shit - because it takes a while to get good at something! - and then the business doesn't take off, and they wonder why....
There are lots and lots and lots of crafts that are huge sellers. Pick one that you enjoy, get good at it first by practicing, and then - and only then - move on to selling. It will take a while but it can earn you a great income as a side hustle.