Considering the absolutly HUGE investment in space, time and money you have made towards your train, I suggest you prioritize making a shaddow-station/fiddle-yard NOW!
Many people dedicate around 50% of the trackage just for this. (My own humble layout has 5 shaddow-stations with a total storrage-capacity of 40 trains (16+14+4+3+3)

You can begin "lite", by making a single track that connects to a sliding plate. Maybe 4 meters long (or whatever length you need in order to accomodate your normal length trains. You can then line up 10 4m long paralllel tracks close together. It shouldnt need more than 50cm depth. -So if you can find a 4m*1m space, you can have this sliding shaddow-station with parked trains.
You slide the table with the hand, and lock the track in place with a pin and a hole for each track.
If you are up to the challenge, you can make the table as a VERTICAL elevator, which would save a lot of floor-space.

Thanks Carim for the advice, unfortunately none of the things you've suggested can be put into action:
a,) the layout was originally made of individual modules
b.) at the time I never thought I need a fiddle yard
c.) I can't add anything on the outside of the layout except the area I've shown in the short video
d.) the added future tracks are only servicable for the outside track and other main tracks would be only accessable via the opposite site's turnouts which is miles away.
e.) an escape track on any main inside track can't be build because the lack of accessing a track by lowering the track configuration.
f.) it would need at least be 200mm lower to be able to access the hidden fiddle yard, furthermore the overhead system has to be able to be constructed.
g.) another option could be to build a helix and place any fiddle yard onto the floor.
h.) a vertical elevator could work and I will consider this option but again it only would serve the outside main track
i.) unfortunatley ther are no escape routes I could use from any inside tracks
j.) with the second main line there is no space to add a track which would be able to run the trains underneath the board.
k.) because all tracks are so close together (10 tracks at the Bonn station, 8 tracks at the snow module and 9 tracks on the freight terminal).
l.) for instance by the freight terminal the only track available is the provincial line which we don't need to create a fiddle yard
m.) by the Bonn station its the a dead end track for the HOe line
n.) by the snow module its the mountains in the back with no room to spare.

so all in all I have got a dilemma, what ever I intend to do, it also has to accomodate the overhead system,

the layout is over 44 years old and at the time I've never invisiged I would have so many trains, changes have been made in meantime to ease the running of trains so they don't crash into each other by crossing over sections and this has partly eliminated any future changes to add a fiddle yard., the other problem I can see today is that the layout was build with individual modules but without them I never could have moved the layout in the past (public exhibitions) and the whole idea behind it in the first place was to exhibit it to the public and not on a railway company orientated running schedule or system, so in those days there was never the need for a fiddle yard.
so its like building a bunker but the house was never designed for it.

Although originally I've designed an overpath for both main tracks on the opposite end of snow module on one side , another in the opposite direction, this would have meant that the layout would have had extra boards on each side with a lenght of 4.8m and a width of 2,4m having enough room for a fiddle yard including a loop, the overpath still exists and to make it operational I would have to widen the existing new garage by at least 2 meters and still leaving a walk way of 600mm between the existing layout and the new extension,
but the escape route would only serve trains on the left hand side running from right to left (this only includes track 2 (inside main line) and the mountain track 4.

there is a possibility to extend the garage on the left hand side but not on the right hand side, its a 1.2m boundary restriction.