I have a 30kg e-bike and I am planning to get a trailer (ibex) for a longer journey. The total weight plus luggage will be 142kg (65kg (me) plus 30kg (bike) + 12kg (trailer) + 35kg (luggage) ).
What about a recuperating brake mounted on the trailer? Is there any small and light-weight system available or can it be build?
A long descent, e.g. 1000m and more, should give some energy back and help to brake.
The ideal would be a dynamo that act like a brake, i.e. offering adjustable recuperation so it works like a brake but creates energy.
As I have 3 batteries, two active, there shouldn't be a problem with charging. (it's a Bosch that needs the 5V trick ...).

You mean a generator on the trailer wheel. Say it is always on and working, generating power while you ride. The power you get back from the system is always equal to the extra power needed to pull the trailer, You won't go any farther than you would have without it.

Now if you use a powered motor, it's the same except when you brake. If you have a circuit that puts the motor in regenerative mode, it will brake your bike/trailer. The energy that would have been used in your regular brakes is sent back to the battery, That's the reason for using a big direct drive motor with regenerative braking in a cargo bike. It recovers some energy that would have been wasted as heat.

Justin from Grin Technologies in Canada (grin.ca) has run solar powered recumbent ebikes with regen across the Alps from France to China knows his stuff. He says you get 10% more range in the mountains,. He will sell you motors you can put in the trailer and the electronics too, Probably cost around a thousand dollars USD, Read his web site and see if it makes sense, I think he's an honest guy.

Me, I don't need the range. I like my ebikes simple and light. Regen doesn't work for me at all. I like going faster downhill and coasting as far as I can before I need to start pedaling again,

The term is 'regeneration', not 'recuperation', and the returns are very small. It was a big deal, and selling point even, when e-assist was a novel idea, but how many regenerative systems do you see out there now? It's not because it's difficult to do. All the raw material is right there but in practice you return so little energy back to the battery that it just isn't worth it for most. If it isn't worth it for the primary drivetrain, it absolutely would not be worth it to source an additional motor/generator to drag (literally) around just for the 'free' energy from braking and downhills.

Regen stuff is an absolute waste for e-bikes and other small lightweight vehicles. You need some mass to get some actual power so in the end you are just expending more effort to get a drop of electric power.

What I would recommend is stopping places taking a break maybe grabbing some lunch and charging while you eat. If it is a dual battery system you will be able to charge a couple batteries at once which will help. It won't be perfect but it will help a bit.

What I would recommend is keeping trips to 1/2 the range of the battery so that an out and back can be completed with 20% capacity remaining. 10% in a pinch. EASILY done. Most people will never come close to exhausting a battery in one days riding.