Hi Rachael. With Maya being 20 months now, then I definitely wouldn't go back to 2 naps.
You mentioned giving her a "small snack" before she has her nap, which I assume would be her one and only day nap? Do you think giving her an early lunch or something more subtantial than just a snack might mean she will sleep a bit longer with a fuller tummy? Can you post your routine with snacks and meals included? I usually feed the boys lunch around 11.30 a.m. with a view to having DS2 in bed by 12 p.m. Morning tea is generally around 9.30-ish but my guys are big snackers, so they usually have 11ses as well
What sort of eater in general is she? It's a long shot, but could it be possible she's waking early from being hungry? That's why I asked in my last reply how she eats, particularly the evening meal, and whether she wants food immediately upon waking. If she does wake at, say, 4.30 a.m. can you tell me what your routine is then? Do you get her up and go straight into breakfast, a milk drink, play, what? Have you tried just giving her a little sip of water in case it's just thirst and then resettling back to sleep?
Another thought: is she still in a cot by any chance? Not wanting to complicate things even more but
some people do seem to have success with changing sleeping patterns when they put their lo into a bed.
On a day when she's up super early and takes a short nap, have you ever tried a really early bedtime of like 6 p.m.? Just wondering if it's worth seeing how she goes with a much earlier bedtime and trialing that for a while, helping her catch up on some sleep. She's waking early anyhow, so even if she wakes at the same time but is getting an extra hour from the earlier bedtime, that's already an hour she's caught up on iykwim.
I don't know how hectic your lifestyle is, or even whether you work or she's in daycare or not, but it might also be worth having a couple of "home days" to try and get into a bit of a routine. Have a really stimulating morning, weather permitting of course, and definitely no trips in the car during which she might fall asleep and put everything out. A couple of snacks during the morning when she starts getting weary, lots of drinks of water, a substantial early lunch with or without a milk drink, and then a good wind-down before her nap. Hopefully then she might take a longer nap which will push the day out a bit - still aiming for an early bedtime. She has a lot of catching up to do!!
FWIW (again recognising they're all different) this is our routine. DS is 21 months:
6.30-7: wake, water or diluted juice in sippy
7.30: breakfast
9: go out, run earrands, playgroup, baby gym, shopping, whatever
9.30 - 10.30: morning tea usually whilst out
11.30 (closer to 11 if he wakes earlier): lunch, quiet play while I clean up which gives him a chance to do his poo!
12.00: nap
2.30-3: wake and drink, play
3.30: afternoon tea, play
5 - 5.15: dinner, dessert, play
6: bath
6.30: bottle
6.45: bed
He is usually cactus by bedtime and asleep within 5 minutes.
Just out of curiosity, when she took the 1.40 nap (which is fantastic!) did you put her to bed later by any chance? I think some ppl do fall into the trap of thinking that they had a longer day nap and therefore won't be ready for bed at normal time. In her case it sounds as though she might need to have a permanent earlier bedtime, perhaps 6.30 or however early you can manage. A btc routine has always worked with my boys. I've had my mother visiting recently and several times after lunch she's said to me "There's no way that boy is going to go to sleep - look at him, he's full of beans", which he might appear to be, but I know he will always take a good nap around midday. I literally tell him it's sleepy time, give him his sooky blanket, take him into his room, change his nappy and pop him into bed. It might just be a question of hitting upon Maya's magic nap/bedtime, iykwim.
Anyway, just some things to think of. Good luck!