I was a thumb sucker, and my 18 monht old has been from at least 4 mos in utero. Once a thumb sucker, typically always a thumb sucker.
I prefer a paci bc you can throw it away and never look back (my son "gave it to baby Jesus" at 18 mos at the church living nativity and it was never requested after that). Unfortunately, with a thumb you can't cut it off. And let me tell you from experience, once she's old enough, don't bother with the pepper or bitter polish you coat the thumb with - one gets very used to it. socks don't work.
Maria's thumb sucking draws alot of attention (since it's so unusual nowadays) - but mostly from adults who were thumb suckers themselves. Several adults I've met (including myself) had dental problems that were CORRECTED by the thumb sucking (I grind my teeth and my teeth naturally fall inward thus the thumb sucking prevented both - and kept my teeth straight).
Despite EVERYTHING my parents tried for YEARS, I didn't quit until I got braces. I got a permanent retainer, top and bottom, but learned to suck through that - and then two years later got braces and only then stopped because it was so painful.
This is TYPICAL of most adults I speak to that were thumb suckers. The average age for quitting is between 11-13.
My advice: ignore it. Despite many opinions, there's ultimately nothing wrong with it and you'll just torture yourself and your child trying to get them to quit. Eventually she'll stop on her own in her own time. Even my pediatrician says not to worry about it - it's a natural thing that provides comfort. And honestly, as far as we're concerned, our kids are going to need braces anyway. But, if you can train her back to the paci, go ahead and try as long as you don't intend to let her have that til she's 6....
But ultimately you have to do what you're most comfortable with, despite what your parents, in-laws, or any other know-it-all old people have to say about it. Just smile kindly, thank them for their wisdom, then promptly discard it.
Good luck!