I think the crux of the problem is that the tank didn't go through a proper fish-less cycle which is actually the preparation of the
filter to be able to process the fishes' waste; preparing it involves dosing the tank with ammonia (which simulates the fishes' waste) for several weeks until it reaches the stage where it is able to effectively convert the ammonia to nitrite and then nitrates -
https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/fishless-cycling-how-to-do-it/. If you haven't done this, and instead just let the tank sit empty (+/- decor) with the filter running, then this is just going to give you more-or-less the same water quality results (for ammonia, nitrite, nitrates) as you would get from tap water although this may to start increase now that fish are present and the fish-in cycle is beginning.
As you now have fish, though, do
not add an ammonia source to the tank that the fish are in as the fish will be producing this - it's best to follow the method for a fish-in cycle
https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/fish-in-cycling-with-fish-how-to-do-it/ and
https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/seeding-a-fish-in-cycle/ which you're inadvertently undertaking.
Keep monitoring water quality levels in the established tank where the gouramis have been moved to - that does sound like a sensible move for them to be in there for now, in light of the immediate improvement in their behaviour, until the bacteria levels establish themselves in the 220-litre tank. Decor, lighting (is it a lot brighter than in the 60-litre tank?), etc, as AtM suggests, are worth addressing in the 220-litre tank - and your suggestion of the addition of more plants and rocks sounds sensible.
[Edited slightly to update.]