My comment for you in an other thread :
https://www.mobileread.com/forums/sh...6&postcount=17
Forget the M92 (that I have), it's an too old device today. I've ordered a Max2. If the price is the main criteria, take the most recent device that you can.
With Onyx, it's always the same thing, the old devices are forgotten, don't expect an upgrade of the firmware.
With the Note (549.78 €) or Max2 (798.89 €), many problems are solved like the annotations. And we can, seriously, expect some upgrades.
For your PhD, you have many years of work. The Note seems really adapted of this type of work, I invite you to read the others thread. The choice of a device of the N/M... series is the SD-Card.