Custom-made military-style jacket (along the lines of the M65 jacket), with four big bellows-style pockets on the front and a game pocket in the back. Also add in lots of extra pockets, both internal and external, for general utility - after all, it's custom-made. You can do much the same thing in a longer trench coat style, although it will be a bit more conspicuous.
You can easily put an A7r3 or A9 body in each of the lower front pockets. Teleconverters can go in the upper pockets, while you can put a 70-200 or 100-400mm lens in the game pocket. Also put your memory cards and spare batteries somewhere in the jacket. Even carrying all this, the jacket shouldn't look too out-of-place - heavily-loaded, for sure, with full pockets, but not looking over-stuffed to the point of being ridiculous. Take it off as soon as you board the plane and stash it in the overhead space.
For the backpack, make sure you get one with a removable camera compartment that can be zipped closed (e.g. Fstop). If the removable compartment doesn't come with a handle, have one attached. When flying, just carry the gear in the removable compartment, like a briefcase - no need to count the extra 1-2kg of backpack towards the weight limit.
Make sure you know the airline rules about 'personal items' and be sure to abuse it - most airlines will let you carry 1-2 items of hand luggage, each weighing 7-10kg (depending on fare class and airline), as well as one 'personal item', which may be a laptop computer, camera or other similar item. For me, that 'personal item' has occasionally been a 500mm f/4 lens...
Filter holders, filters (in a storage case), tripod heads and other heavy, solid metal objects, chargers (without batteries in them), etc. can all go in checked luggage - they may be expensive, but should still come in below the limit for travel insurance.