Bromptons are heavy. They are compact and carry luggage well. But they are heavy. An aluminium framed Dahon with 20” wheels will not be any lighter.
The decathlon is fine for a short commute. If you are just jumping on a train (not tube) most people seem happy with the simple fold in half. Rather than collapse down the bars and saddle. So fold is really not that important.
I’d buy a singlespeed decathlon, strip it to be as light as you can, use that for a while and see how you like it.
There are no light folding bikes. My aluminium framed 16” wheel Tactic is just 8 kilos. It’s hard to go lower without titanium, no mudguards or rack and no carbon parts. If you get below 10 kilos, that’s not a bad target. My old Brompton was more than that without the posh parts.