I'm still mulling over your route options.
Burgdorf and Bern as transfer points both have advantages and disadvantages.
Bern has a much greater capability of tourist services. The luggage office at the train station is open later, more choice of rental cars, no deposit. (BTW, I'd pick Avis, because of it's proximity to the Autoroute/ Expressway and easy routing to Rüegsau) . Your rental car almost surely will have a GPS available, useful in cities that are not built on a rectangular grid of streets. You will have a key drop, and a range of vehicles available. SUV's are not so common as in the USA.
Burgdorf is closer, and you can get off at Burgdorf as you come from Maastricht. (I'd leave at around 7AM, but the trains around 8AM in theory would work. I always like a time cushion for errors.)
The rental car situation bothers me. It has the possibilty of misunderstanding what you will get and whether it will hold all your luggage. If you leave luggage at the station, you have much tighter time constraints, unless you find a locker/lockers.
You have multiple changes on any train route from Maastricht, , and I'd pick the fastest route to get you to Switzerland as early as possible. Take a nap on the train, if there is any segment which lasts long enough, or one of your party watches for the transfer points.
By using the SBB website ( which you can use for any station once you find the place in the menu called "find a station") and looking under Services tab, I find that on Saturday, the luggage office at Burgdorf closes at 1730. If you look under the Equipment tab, you find that there are exactly 6 Large and 12 small lockers, accessible 24 hrs per day.
If I check Bern on a Saturday,
I find the luggage office open until 1900 every day of the week, and 240 small, 141 medium and 36 large lockers, accessible from 0545 until 0100 the next day
In either station, if you can put some luggage in a locker, which I do routinely, it will increase your flexibility if the SUV does not have enough room for your luggage. At Bern, the Left Luggage office is open late enough (1900) to definitely work for you ( I think.) Maybe not so at Burgdorf... 1730.
That is an example of what I mean by working with the system. Bern is equipped; Burgdorf is marginal.
Possibly, pick up on Fridday in Burgdorf would have time constraints. Bern (especially Avis) probably has longer opening hours. I mentioned it before....I think until 1830.
The biggest problem with Bern would not be the ride to Rüegsau; rather, it would be finding your way back to Avis ( or other) when you return. Burgdorf would be easier, if you know exactly where to turn. A GPS would minimize the problem in either town.
The second biggest problem would be that coming from Maastricht, you would need several extra minutes on the train to Bern (15-20 minutes) , plus a longer drive to Rüegsau.
If you have to haul your luggage to the rental car location, I'd try to find some way other than public transport. In either town, in spite of the expense, I'd consider a taxi. It would be great if they could pick you up.
If you can leave some luggage at the station, then it would be easy to use public transport to Avis or Sixt in Bern, or you could still take a taxi. (Don't know about Burgdorf). 5 people in one taxi is possible only if you can get a van, not the normal car. It is possible. With luggage, it is harder, but not impossible.
Will all five of you be coming from Maastricht?
Your BnB will be about 10 minutes and 6km from Selvam.
From Avis at Bern, It will be about 35 minutes via Worb, then a good secondary road via Walkringen and north to Rüegsau.
Balancing all those items is a difficult exercise.
You have trade off time, money (taxi?) and "quality of result." (Meaning ease of getting the car and having to haul luggage, as well as (I believe) greater certainty due to longer opening hours and dealing with a larger rental car vendor with no deposit if Bern is chosen.)
If you have to return the car to Selvam by, for example, 4PM, and if they do not have a key drop (?) you may have lost enough time in the Emmental to make up for travel time to/from Bern.
if you are going to get your Berner Oberland Passes in Bern, I think that you will get them at the ticket window (open late) , rather than via the moderately easier process at the SBB travel bureau (Reisebüro) nearby, which will likely be closed ( at 1700, I think, uncertain) by the time you get there.
Add to it the uncertainties of being in a new country and tired; even though a rain ride can be restful, it is not when you have five train changes and haul luggage.)
Everyone has there own perspective. If I were comfortable with the rental car situation, especially car return, in Burgdorf, I'd go that way. If I were worried about the rental car issues, with 5 people and luggage, I'd select the more inconvenient but surer Bern option.
That's me; might not be you.
It certainly focuses the whole decision process around the rental car. I note that you have recognized that; you have been working hard on it.
So, what if the rental car at Selvam works well? Great. Definitely the way to go. The earlier that you can get there and sort that out, the better. There are trains from Maastricht departing at 0657 and 0711 that get into Burgdorf at 1506. I'd try to get one of those if i could get up in time in the morning.
(use the SBB Timetable - www.sbb.ch/en/home.htm l
What if you get off at Burgdorf and find a problem? At 4PM (1600) on a Friday (actually 1630 before you figure out the problem) it is not completely impossible to go on to Bern by train and get to Avis by taxi by 1830, but it would be an adventure. If you get into Burgdorf at 5 PM (1700)....not possible. All the better if you get to Burgdorf at 1506. If you can do that, you give yourself some useful flexibility.
You could rent two cars at Selvam, trading off cost for convenience with luggage, if not with any change in any return time constraints. Rüegsau is close to Burgdorf, but the traffic on the roads (though not heavy) is not fast.