There are addons that allow you to unlock the framerate cap. 100 FPS seems like a weird place to artificially cap if you ask me. I have a 200hz Ultra wide monitor and 3080 TI. I typically cap at 144Hz. I think most people are kidding themselves if they think they can tell a difference much above that TBH. I can run very stable with my setup in ultra settings @ 144 hz @ 3440x1440. I have gone as high as 180, but the game is not quite stable that high. To my eyes, 144 locked looks better than frames bouncing from 165-180.

Whether or not you can run 165 comes down mostly to your GPU, desired graphics settings, and Resolution of your screen (ESO specifically is also fairly CPU intensive). I can easily run 200plus frames in 1920x1080, but couldn't do that in Ultra Wide or 4k. I would only go as high as what you can run stable 99% of the time at your desired graphics settings and resolution. I want to run with max graphics on an ultra wide monitor, so with my setup, 144hz seems to be the sweet spot.

Its essentially a 3 way balancing act. Pick your desired Graphics settings, your screen determines your resolution, and your GPU/CPU will determine how many frames you can pump out. I would cap 10-15% below of your average stable frame rate. If you rarely see 165 FPS, dont cap it there.