I have my beard cut to the bottom of my jawline, so I find it easy to make it straight as I am following that line. I trim my beard using scissors as I find trimmers take it too short and I want a beard and not stubble. I then razor my neck daily when I do my head
About 5-10 years ago i use to do the thin pencil beard and sideburns thing, when i look back at the pics i shudder
Now i prefer everything natural, i just don't like prominent lines on myself.. it does work better with some people though imo.
I think you can either hide the line by cutting it underneath your jaw/chin or perhaps leave the full neck length, then demark the line with with a trimmer and do a short graduation/fade technique to get a nice smooth natural look.
Start with a barber i think and learn the technique and how to maintain it.
Just ideas though, im no expert