Happy to help :)
If you want to do Brienzer Rothorn in August you'll need to get a seat reservation in advance for the time you want. You can wait to purchase the actual ticket until you know weather is good, but you should pre-plan the day with a seat reservation, as you won't be able to just show up and get on the train in August.
Schynige Platte I'd recommend arriving in Wilderswil 20-30 minutes before your cogwheel up to SP is set to depart, as there will be a line and in August it was packed full. If you do SP to First via Daube/Oberberghorn/Faulhorn you'll get amazing ridge views of the lake and mountains (much better than Harder Kulm). You'll need to time correctly to make sure you arrive at First before the last cable car goes down for the night.
I think I already told you about Murren to Schilthorn via Rotstockhutte but if you want to do the ridge version you could go downhill instead and do Schilthorn > Grauseeli > Wasenegg Ridge > Bryndlii > Murren -- this keeps you up on the ridge, as opposed to on the side of the ridge going to Rotstockhutte. I prefer going downhill from Schilthorn to Murren instead of up, as then you are walking towards the main views. But it's a cardio vs. knees trade off. Typically we do Birg > Grauseeli > Wasenegg Ridge > Bryndlii > Murren to save the extra downhill from Schilthorn on the knees.