Hello Thiengairrue73
Your question is related to activities doable in Grindelwald. On this platform, you will not find people having done these activities before. Thus, we cannot give you advice on that.
From the media (newspaper reports), I know that First is very busy during the summer, and if the Jungfrau.ch website (that is operated by the same management as the First glider) says so as well, you should follow their advice.
I suggest that you should post your question in the TripAdvisor Switzerland forum too, which is run by locals and other tourists do comment based on their experience.
https://www.tripadvisor.com/ShowForum-g188045-i336-Switzerland.html
In August, if you start early, you will have enough time as the daylight lasts until 9 P.M.
On the way back from Grindelwald, you could travel via Kleine Scheidegg and Wengen, to end up in Lauterbrunnen. Then cable way up to Grütschalp and back to Mürren.
You can find all the timetables on www.sbb.ch/en.
Please let us know if you have got more queries.
Kind regards,
Roland