We have done the Schynige Platt to First hike twice. We catch the first or second train up to Schynige Platt. It is a beautiful hike and is not crowded until you get to the Faulhorn area. The hike from First to Faulhorn and back is very popular and the trail is quite populated on that part of the hike. Last time we did it, we stopped at both of the huts (one for lunch, one for desert) and still made it in plenty of time to get down on the lift.
We have also done the Mannlichen-Kleine Scheidegg hike twice. It is a very busy hike. Try to do it early in the day as it gets quite crowded mid-day. It's not my favorite as it is almost a road, but the views are amazing.
We have done the hike to Obersteinberg three times and it is our favorite. It leaves from Gimmelwald and climbs steeply up with wonderful views looking down the valley. You walk through a cow pasture and crest a hill. From there you are looking down at the Obersteinberg hut and stunning views of the mountains with a large waterfall. It is a full day hike. Also, I don't like the long hike down to Stechelberg. After doing that the first 2 times, we changed it up in 2018. We hiked back the way we came to Gimmelwald, and took the lift back up to Murren. You could also ride it down to Stechelberg. This was the hike with least amount of hikers. You won't have it to yourself, but it is not as heavily used as the other two.
We will be there in September and are looking forward to hiking in the area again. I understand the problem with picking out trails. I can tell you that all these trails are well marked. We have also hiked in other areas of Switzerland, Austria, and Grainau Germany. PM if any of those are on your radar and you want some tips. For the record: We are 65 and 66 and have been hiking in the alps on multiple trips. We just returned from Andalusia area of Spain where we hike 5 hikes in 4 days.