I just returned from some Yosemite high country day hikes. Mostly, the FR255 continues to be the tool I wanted back when I considered it versus the "outdoors" watches.
My longest hike covered about 20 miles and 3000' elevation gain/loss. That was ~12h elapsed, ~10h "total", and ~8h "moving". I did the "resume later" pause function during the ~2h of sitting breaks to eat, rest, and filter water. But, I did nothing for frequent standing rests on switchbacks etc. The battery discharged about 50% for the whole day. I used the "auto select" GPS mode, since the sky exposure varied a lot with steep canyon walls and trees at times.
I tried to use auto-pause on an earlier hike. But it was ridiculous, buzzing on and off where I had to slow down and worry about footing. It took me about 2 minutes to be aggravated and go back in to disable that option!
In the end, I did not use course navigation even though I prepared courses in Garmin Connect and synced them to my watch ahead of time. I was worried about some reports on these forums of watches crashing on long courses, and I didn't want to risk that. I used a mixture of the NPS app and Garmin Explore apps on my Android phone for map views on the way out. I did use the breadcrumb map of my watch at times on the return, when I was tired and wanted quick assurance that I was on the same path. Mostly, I used a data screen up with elevation, elapsed time, clock, distance, last lap pace, and ascent/descent gauge. I have my watch set to do 1 mile auto laps.
I'm glad I have the large screen and resolution of the FR255. I also carried an inReach Mini 2, but I never felt the need to consult it directly. I think my watch provides better nav aids and activity stats, and the Explore app provides the better satellite messaging interface.
I had a full range of lighting conditions from sun with polarized sunglasses, to heavily shaded woods, to finally using my headlamp at the end. At no time did the MIP display disappoint me. In fact, I didn't even activate the backlight in the dark, because the off-axis glow of my headlamp already made it very readable whenever I brought my wrist up into my field of view. By comparison, my OLED phone screen (Google Pixel 8a) was readable but poor contrast under the brightest conditions with sunglasses.
Also, the altimeter seemed to work great. For my 12 hour hike it has only 10' discrepancy between the ascent and descend figures with the activity started and stopped next to my car at the trailhead (3081' ascent vs 3071' descent). All of my hikes were in the 6300' to 9200' elevation range, and I was driving back down to around 3500' to sleep each night. The readings always made sense when I looked at them throughout the week, and I never did any explicit calibration steps. I synced my watch once each night while charging it, which was also when my phone had service. Otherwise I kept the watch bluetooth disabled.