Don't know how big or heavy you are (?), or how vertical your trails are ... The choice of 27.5 vs 29 has to do with what you have to roll over (mostly), and wheel speed vs handling. Just like in the Motocross world where we all rode 21" fronts for ever, then Husky started playing with 23", etc. By the time we got to Motard, we were looking at 19" and 17" wheels, and maybe smaller to turn quicker ... Larger wheels will feel more stable (a bit slower turning w/ a bit more gyro), but if your technical sections are rally tight, you may want smaller ...
Dropper seatposts are pretty much a must, and none of these seem to come with one ... And good ones are good and expensive, so add that to your budget. Same for forks. If you are doing much climbing, you want light forks so alloy and air is good. Since you come from MX, you know how to tune suspension. Oil viscosity, air volume and pressure, dial in compression and rebound so maybe Fox, Manitou Elite, etc. Fork should be 3.25 lbs or less.
Then it all comes down to frame geometry that feels right to you ... I can't answer that. That's all about test rides. All three brands build really nice bikes. But with all the unknowns here (from my perspective) I'd prolly take the Diamondback (and I'm sort of a Specialized fan) ...
The downside is the cost and then thinking about alloy uppers for the forks. You could ride that one with steel uppers for a quite a while before "needing" to loose the grams. By that time you will have settled on what replacement you really want
The Liv is the least expensive (so you have room to upgrade components), but that Suntour fork would not work for me ... So, I'd factor in $300 for a Manitou in about a month and that is not what you indicate you wanna do ...
The Specialized with the 3X drive train might be a turn off ... You can get caught out trying to shift across that front set. The diamondback 1X is more forgiving from where I stand.