I know this is an old thread, but I came across it regarding what gears to use in ERG mode for my Gen 1 Neo.
I do find that, on the OG Neo, it feels “easier” to ride in a bigger gear, likely due to the increased inertia, compared to riding in a smaller gear.
However, with the Neo 2T, I’ve read that at harder gears, it tends to under-estimate the power output compared to an easier gear. I feel like the fatigue you’re experiencing may be as a result of this. I’ll edit this post when I find the sources for this, it’s been a while since I’ve read into this.
Edit: Around the 14 minute mark of this video: https://youtu.be/9DqiBksaVSY