???
from what information are you getting this from?
There really isn't any need for a DAC unless it's for higher end system, for the most part an AMP with the onboard DAC of your motherboard should be fine.
If it's running Verizon or AT&T software, you cannot unlock the bootloader to install ROMs, you need to have an international qualcomm variant of the phone to do this.
good logic, but I don't think LG is the way to go when it comes to support. Nexus phones are guaranteed 2 years of support, so if you want to stick by that statistic you can, and the 6P is very good.