Day trip from Lucerne to Grindelwald via Bern, is it possible?

That's possible just leave as early as possible.
You can look up the train schedules Lucern to Bern to Grindelwald to Luzern:
https://www.sbb.ch/en
You need to leave Grindelwald at 16:17 to make it back to Luzern by 18:55

It's possible, but it's going to take you around 6.5 hours of transportation, not including any time in the actual places. Also, the actual town of Grindelwald isn't really the "highlight" of the area. To have a meaningful trip there you need to make time to go up to some of the mountain viewpoints, which is going to add another hour roundtrip and isn't included in the Saver Day Pass.

If you want to be back by 7 pm, you'll need to leave by 7 am and you'll have around 90 minutes in Bern and around 90 minutes up at Grindelwald First. Seems like a lot of chewing for not much meat to me.

Something like this....
7:00 - 8:00 - Train to Bern
8:10 - 1.5 hours walking around Bern, some things won't have opened this early, so not really sure if there is much point?
10:50 - Arrive back at train station to leave
11:04 - Train to Grindelwald
12:40 - Walk to Grindelwald Firstbahn station
1:20 - Arrive up at First
3:00 - Depart First to get back to Luzern by 7:00 pm

For only 2.5 days in Luzern, I think your time is much better spent doing an afternoon in historic Luzern, a day doing Titlis or Pilatus and a day doing a boat ride to Vitznau + Mt Rigi. I think you'll enjoy your short time more that way.

I was thinking as have a saver day pass for that day, going to Grindelwald, but not missing to have a quick stopover at Bern as travel time to Grindelwald is the same. If it's too much, we can skip the Bern part and go directly to Grindelwald and spend a good time there. Got a suggestion that Grindelwald is very beautiful and worth to visit there. Thanks.

if you do as you plan, your train to Bern will be either the slow bucolic train through Langnau im Emmental or the faster main line via Olten. Either way, while the scenery is alright you will miss the beauty of the scenic cogwheel train directly linking Interlaken with Luzern over the Brünig Pass, one of the most beautiful routes in the country. You may go over that route on the evening trip back to Luzern but I expect you will be tired enough of train travel to not notice what is outside the window.

How's doing the opposite direction? Start from Luzern, going to Interlaken - Grindelwald. Enjoy the day there, if time permits, go to Bern and from there to Luzern- is it doable?

Yes, this is certainly doable! You have come all of the way to Switzerland, I do not see why you would now limit yourself once here. A 2.5 hr train ride each way for a day trip is nothing - especially when considering what you will see out the window on the way.

Doing the shorter, scenic route first is a great idea. Then decide to add Bern, or just head back to Luzern the way you came. Equally beautiful in the opposite direction!

You could also easily switch your days around. I would base it all on the weather. It is not quite May yet, but the weather is just stunning right now, I hope it will be that way for you!

Luzern - Grindelwald 2.5 hr

Via Bern - 3 hr

Luzern - Titlis 1hr 15 min

Use the MeteoSwiss app

In it you can save multiple locations

Choose:
Titlis

Grindelwald

Maybe Lungern, which is along the scenic (shorter) route to Grindelwlad

Bern

Also - bookmark webcams for Titlis
and Grindelwald village, such as this one:

https://belvederegrindelwald.roundshot.com/#/

Look the day before at the forecast for each location.
Look again first thing in the morning. Check webcams.
Go where you have the best chance at good weather.
Enjoy!

If there is something you really want to see, I would certainly go, why not? One year we took visitors from our home in the BO to Locarno for a day trip - 4 hours each way. This was years and years ago now and it is still a memory held dear by all of us to this day!

that may well be a better idea. Sit on the right leaving Luzern and turn around but stay on the same side when the train changes ends in Meiringen (home of the easily walkable Reichenbach Falls of Sherlock Holmes fame). Enjoy.

When you have had enough of the incredible mountains in Grindelwald (although I might prefer Wengen and Mürren personally) you can decide if you want to add in the trip to Bern (do you now which of its many attractions you want to experience?) and then the fast train back to Luzern. Or retrace your steps and see the Brünig Pass route from the other side.

both writing at the same time

Thank you all. I will do that, first want to take the train to Grindelwald, if time permits, will make a stopover on our way back to Luzern.