My OH had a Cotic RocketMAX and really loved it. She reckons it was the most confidence inspiring bike she’d owned (& she’s owned a few!!). The Cotic was stolen last year and she ended up using the insurance money to get a Trek Rail, which is the ‘e’ version of the 2020 Trek Slash. She’s comfortable tackling much trickier terrain than she’s ever attempted riding before – even on the Cotic.
I just put this down to the extra weight and chubby tyres on the Rail, but I rode it quite a lot while waiting for my insurance replacement. Having tried a Trek Slash as well, I think more of the confidence comes down to the geometry – its really good! For the high level of confidence and capability on steep and rough stuff, it’s still feels nimble and easy to navigate through technical terrain. Feels perfect for off-piste stuff at GT and Inners. On slippy twisty trails like ‘mud and roots’ at GT, it’s in a different league to my old RocketMAX.
I liked the Rail and Slash so much I ended up getting a Rail. Still missed my analog bike so bought a budget 2020 Fuel EX for a contrast. The geometry of the EX is also fantastic and very confident on steep stuff. However, for what your OH is looking for, I reckon a 2019/2020 Slash would probs be better. If you’re lucky and not in a rush, you might be able to find someone who’s changing their ‘old’ Slash for the new 2021 model – if you can get a carbon frame version, they are ace😋
FWIW, my OH is just under 5’10” with 33” inseam and she’s riding a large Trek Rail with 50mm stem (& I reckon she could get away with a 40mm stem for steeper trails).
HTH