Very conflicting sentences, but eyestrain is very individual! You gotta play around with settings/monitors to find what's best for you, sadly. There are some guidelines however.
Brightness is dependant on the environment you're in and it can never be "super dim" if "it feels best at" for you. Adjust accordingly. If you enjoy 60 nits (cd/m2), use that setting.
How come you're not using the setting you're comfortable with and instead use higher brightness? That's a recipe for disaster.
If you can, try finding the optimal contrast setting (either with your colorimeter or browsing some settings online from some reviewer) and just adjust the brightness to your environment and what works best for you.
Consider investing in blue light glasses with prescription instead of using monitor's OSD and f.lux (try using both if that's what you prefer, but the effect won't be the same as using actual glasses) IF you are certain that blue light is the cause of your eyestrain.
One thing to consider with your particular model is the curve. Some people have mentioned getting eye strain from very curved monitors (1000R curve on 27" being a culprit), so this could also come into play!
Considering the monitor is Freesync and DisplayHDR400 certified, it is flicker-free and 8-bit (without FRC) confirmed, which could be good IF you experience eyestrain from PWM and temporal dithering (the latter being a very small % possibility)
Another possible culprit could be with the way motion is handled on your display. It's a VA panel, which is notorious for black smearing (dark transitions aren't fast enough). This can also cause eyestrain for some!
There's also things like polarizers, anti-glare coatings, LCD inversion artifacts, OD setting perhaps not tuned (easy fix, just run TestUFO and check blurbusters' site on how the testUFO should look like) right etc.
You should probably mention previous displays which worked for you without any issues, so your journey is easier to track.