Murren & BO: 2 "too" questions

1. “Too long” is very relative. We spend our summers here in Mürren every year and never tire of it. Someone else may say it’s too remote or not enough shops/restaurants. Very much personal preference. If your priority is hiking and scenery, it’s great. If your priority is day trips to the region, it’s not convenient for that.
2. Some hiking trails and scenic cable cars/trains don’t open until late June/early July. If that doesn’t bother you, it will be a quieter time for sure, whereas summer tourist season here (July-September) is absolutely nuts now. Swarming with people unlike in years past - the cable cars are packed to capacity.
3. It definitely doesn’t make sense to use Murren as a base to see a place like Luzern. If you want to be up in the mountains, it’s great. If your priority is seeing as much as possible and are “on the go” people, it’s not the best option.

5 days is fine for Murren if you like what it has to offer. I recently stayed a week and it was just the right amount of time for me. However during that time, I hiked Schilthorn, Mountain View, North Face, Obersteinberg, etc. I also did the via ferrata and paragliding, and spent a couple days just chilling. If you don't like to do these things, I'd suggest a day trip to Murren instead of spending 5 days.

Murren is generally not a good base from which to take day trips. It takes a lot of time to get anywhere and you will have to use multiple modes of transportation. Even getting to Wengen and Grindelwald can take a long time. For day trips to Lucerne etc, I'd base myself in Interlaken.

I would have loved 5 nights when we were there in mid-June for 3 nights, but quite a few trails were not open yet. There was still plenty to do, including hiking the North Face trail and hiking in the Lauterbrunnen Valley and seeing sights on Lake Thun and Lake Brienz (although I'm not sure how long it will take the town of Brienz to recover from the recent storm damage). However, as others have said, it's not a good location for day trips to Lucerne.

You started your post listing Murren so I assume your priority is mountains (mine too). I found the BO magic started to fade after Lungern and spent only half a day in Lucerne. We spent 7 nights in Interlaken and that was perfect for me as we also ventured to the Kandersteg and Meiringen valleys. My following trip I stayed in multiple locations and wasn't a fan of the toting my one bag and coordinating drop off with the next lodging location.

I wouldn't look for anything beyond Spiez or Brienz. You can determine approximate travel time with Google Maps (using your travel dates) in your planning.