Light aircraft pilot reported an NMAC with another light aircraft in the vicinity of S43 airport.

Narrative

I was on a training flight working on Commercial maneuvers. I departed S43 (Harvey Airfield) in Aircraft X at approximately XA:45 AM local time; departing the airport environment to the north after a takeoff from the paved portion of runway 15. Winds were light and variable on takeoff as well as at 1500 feet AGL where I leveled off. I proceeded to an area approximately 9nm north of PAE on a 016 bearing where I set up to practice 8s on pylons over a slough in that area so as to avoid any congested areas. Upon completing the maneuver I entered a climbing left hand turn with the intention of determining what the approximate cloud bases were and attempting to complete a chandelle. I exited the 8 on pylon at roughly 900 feet AGL/MSL (terrain was nearly sea level in that area) on a heading of 245. Upon reaching approximately 1800 feet MSL on a heading of 080 I briefly rolled wings level to scan for traffic and saw the conflicting traffic appear out of my left wing strut roughly 50 feet below and maybe 200 feet in front of me. The conflicting traffic appeared to be Aircraft Y and was close enough that I could clearly read the tail number but was so close that I did not have time to fully absorb what I could read. The traffic was on an almost exactly perpendicular course to my heading upon roll-out; so I believe that my lowered left wing blocked my visibility of him until I raised that wing to conduct my scan. I do not believe the traffic ever saw me as they made no attempt at evasive maneuvers.There were several other contributing factors to this NMAC:1. My choice of maneuvering location. This location was very close to I-5; which is an easy reference to use for aircraft transiting the area heading from north to south.2. Failure of the other aircraft to see-and-avoid. Given the proximity of the pass and the fact that they continued on what I assume was their existing altitude and course; I do not believe that they ever saw me. I should have been much more visible to them than they were to me.3. ADS-B failures. My aircraft is equipped with ADS-B Out and ADS-B In. The ADS-B is displayed on a Garmin GTN 750; but this traffic was not displayed as a Traffic Alert to me until AFTER I had made visual contact with them. I have no way of knowing whether the conflicting traffic had ADS-B In; but they were within just a couple of miles of the SEA Mode C/ADS-B Out veil and I did eventually get a traffic alert from them; so I highly suspect they that were at least equipped with ADS-B Out capabilities. I am highly suspicious that somebody at the flight school may have changed the default settings for which traffic is displayed on the GTN 750; as there were several other visually confirmed targets that were also not showing on the display. Distance didn't seem to be as much of a factor as on my way out to the practice area the GTN 750 showed me a target that was -100 feet at 2 miles but was NOT showing me other aircraft that were in the traffic pattern at S43 while I was holding short to depart.