Let's look at what the OP is carrying:
GFX 50S - 920g including EVF, single battery and memory card
23/4 lens - 945g excluding hood and caps
32-64/4 lens - 975g excluding hood and caps
110/2 lens - 1010g excluding hood and caps
That's 3850g so far, and that's just body and lenses.
D4 - 977g
He didn't mention which tripod, but let's assume the RRS TVC-34L (that, or a similar Gitzo, would be fairly typical for this kind of high-end, non-ultralight setup utilising a D4 head) - 2146g (assuming no levelling base or other tripod accessories).
Now it's up to 6973g.
Add in another 1000g for typical accessories (spare batteries, charger, cards, quick release plates, cleaning gear, remote release, CPL, solid ND filter). With a full filter system and L-plates, this could easily reach 1500g.
Add in another 2000g for reasonable non-photo gear for a day (snacks, water, spare/bad weather clothing, tissues/personal hygiene).
Then you have the weight of the bag itself - probably 1.5-2.5kg, regardless of whether the bag is good or crummy.
You're dealing with 11.5-12.5kg on your back. Although not particularly heavy (hikers will carry a lot more), it's not insubstantial either. You can get away without a proper pack if you're just carrying it 20-30 minutes before putting it down to shoot from a static position, vehicle or other place where you're no longer carrying it around, but you wouldn't want to be carrying it on your back all day without a well-designed, load-distributing pack.
Neither the Flipside Trekker 350 nor the Think Tank Urban Approach 15 has a proper frame (the Lowepro has a frame, but not a rigid one that can bear weight; the Thinktank doesn't seem to have one at all). The Think Tank doesn't even have a waist belt, so couldn't distribute the load even if it did have a frame. Fine for dragging gear short distances, but your shoulders would be killing you if you were carrying 11-12kg around all day.