>>2211311>>2211552gonna further add to my first post. Since you're looking at Helikon stuff, you could consider getting their numbat chest pouch and use that to carry around day-use stuff when you leave your main bag behind. I've been eyeing that thing personally.
If you really are looking for modular stuff, there's other setups like the very expensive Hill People Gear packs. Their stuff has a separate hip belt which you can attach and detach to the pack, and use as a standalone belt. They also make a pack frame that works with the belt and can hold different size pack bags, which you could switch out at camp if you were so inclined. Here's a video of their bigger packs.
https://www.youtube.com/watch?v=E-CyYAwZKuUOn the other end of the price range, at least in the US, there's some inexpensive molle packs made by LA police gear if you really want something with a bunch of molle on it, but aren't sure if you want to drop a ton of money for it. They also make a chest pouch attachment for their Atlas line of backpacks, but you'd have to be ok with wearing a chest pack that looks like tactical manboobs. I have some of their pants and pouches and it's overall good for the price.
If you aren't dead set on modular stuff, I own a mystery ranch scree and am very fond of that pack, though I'm not sure I'd personally do multi day stuff with it like you're looking for, but you could look at their stuff.
