I think I am reading between the lines, that you may not think it is easy to keep the small tanks — and I admit, with other species I may also have had the experiences of endless problems with waterquality, too much food, etc etc — but really in paro-aquaristic you will find that it is not really near as problematic.
Breeding paros mean you have one tank with two (very small) fish, male and female. You dont feed of course dry food, or frozen food, — you feed live food, and you dont feed very much (thats an important point of course). Paros dont need a lot of food. So you dont have heeps of too much food rotting. Even the fry, you feed them microworm for example, — in a small tank they will find the microworms even if you dont overfeed the whole tank.
If you need to change some water, you use a small tube, you dont of course vacuum the bottom at all, you change water carefully and its fine.
In a small tank once you see the male in the cave with the eggs, you can more easily catch the female out carefully, and then the male with stay with his duties untill the eggs have developed and after a week and a half, he will no longer be needed.
From that point on, even in a small
12 liter tank, you will think you have no fry at all. You dont see them, for weeks perhaps, they are that small. But they are probably there, — and you just keep adding a little bit of microworms, — and after a few weeks or even before, a little bit of artemia. Perhaps after two weeks one day you suddenly spot one
2 mm. fry sneaking out to get an artemia.
In a big tank, — you will never see the fry. They may of course survive, a few, — then you will suddenly in two months time see a
1 cm long fish, and you will be quite surprised, because you were certain there werent any.
Perhaps you can take out the cave from the bigger tank and transfer to a smaller, perhaps. I would think that was a risky operation.
And

… I dont want to continually take away your optimism, but to catch a paro in
240 planted or decorated tank —

…
I would — in your case — as you are getting fish from Bernd, have a seperate certain pair, put these into a small tank, — if you have no space you can have them on top of the big tank

.. I do that because of lack of space. Try with the rest in the big tank and see how it works there. Then when you have fry from the couple, put the pair also in the tank if it works well.
Kind regards Helene