The highest string (the last one to the right) seems to become flatter as I move higher in pitch. I know this is an intonation issue and can be adjusted but not quite sure if I should adjust from the neck or the bridge. Is there some sort of guide for intonation that someone wants to share? Thanks!

the bridge.

on modern stick bridges you'll notice that the saddles (the parts that receive the strings) can move back and forth thereby lengthening/shortening the string as needed.

since the frets are fixed position, what we do is alter the length of the string to match up with the frets.

OK, if you get flat going up (in pitch) a string -- it would mean that the frets aren't "eating up" enough string (making it short enough) or - in other words, the string is too long for the frets.
so we need to shorten it.

but by how much?

well, the old trick is to use a point of physics - the "harmonic". a harmonic in this case is where we touch the string (without fretting) to create a node (a stop point) not with the fret, but just relying on natural resonances. This is fixed by the physics of the string - so we can use it as a reference point for the frets.

here's a guitary explanation
https://www.youtube.com/watch?v=8dXhFWmooHw

here's sort of a physics explanation
https://www.youtube.com/watch?v=RUpjYDteYcg

now the one we are interested in is the 2nd harmonic (the 12th fret harmonic) which happens at the half way point -- since an octave is
a doubling of frequency, that the octave

so basically what we want to do is match the 12th FRET note (which we can change by moving the bridge saddle, changing how much %-wise the frets eat up the string) to the fixed 2nd (12th fret) harmonic

now these are very fine adjustments,so I'd strongly suggest using an electronic tuner.

You can see Greg Howard walking through this

https://www.youtube.com/watch?v=v1ZLQG4_A3c&t=285s

the intonation adjustments start about 15:00

Cool! That's very clear and helpful. Thanks for the informative post and the videos!