I can feel for you. We have ultrasound pics from all 3 of mine sucking their thumbs. The first 2 don't any more, but 3 is a baby so we'll be dealing with it for a while.
I know a lot of people who don't care if their child sucked their thumb, but they mostly only sucked at night. My oldest sucked his thumb around the clock. If I wasn't interacting with him, or he wasn't eating it was in his mouth. We tried a few things starting around age 3. We told him if he'd no suck for a month he could have a dog... well that was awful. He wasn't ready to stop, but he wanted a dog so badly. so he'd suck, and then remember and then cry that he wouldn't be able to get a dog. After 3 days of him barely sleeping (because he needed his thumb), and crying we stopped for a bit. We tried a bunch more things, but finally when he was 4 1/2 we did it.
I think it was a combination of him being ready and the incentive we did. We started with telling him that he would get a nickel for every time of day that he didn't suck his thumb (bfast-snack, snack to lunch, lunch to snack, snack to dinner, dinner to bed). Initially that was hard enough that he'd usually get 3 nickels a day. I'd give him the nickel as soon as he earned it so it was an immediate reward. And as soon as he had 1 dollar I took him to the dollar store to cash in on the reward. (After a while that meant a dollar store trip every other day, but I was willing to do it)
After about a month he was going the whole day without sucking his thumb, so we just switched to if he didn't suck it all day he got a quarter. And we told him that if he didn't suck it at night he'd get 1 quarter. After another 2 months he was pretty consistent about not sucking at night too. We spent a bit of money, but it was so worth it to break the habit.
One key thing was that we never got after him if he did suck his thumb, his consequence was not getting the money.. and after just a few visits to the dollar store toy section he realized how much he wanted to earn that money.
I don't know if it helps, but it was our only successful attempt. Good luck!