We managed extended periods off grid with 2 x 3.7kg bottles set up much as in you photos. Gas was only used for cooking & the two bottles could last us several months. If you are using gas for space heating and or hot water you'll soon find how long you can last on two 4.5kg bottles & can adjust you time off grid accordingly. Just means you have to refill more often than you have been, which isn't really a big deal. It was for us because so any places have changed to swap & go, who didn't have our size bottles , so we mostly needed to find places to re-fill them.

Our new rig has no gas at all. Hot water is heated by either the vehicle's engine or via a diesel heater. Space heating similarly by engine or diesel heater. Cooking is via a diesel cooktop, a single plate induction cooktop & a combined microwave/convection oven.

We'd heard some terrible tales about diesel cooktops, but so far we have been pleasantly surprised by how good it is. Runs off the vehicle's fuel tanks. Havent tried the other methods yet. Not sorry to ditch gas though. .

Something to take into consideration is that the smaller gas bottles can be relatively more expensive to fill, and some swap & go places only carry the 9kg (or 8.5kg as a lot call them nowadays). I recently had to get our spare bbq 4.5kg filled and it was about $18 ($4/kg) where the 8 5kg was about $22 ($2.58/kg).

We had the same for weight savings, be aware when you are travelling remote, that the swap and go for them is hard to find and refills not always available. So plan you refills for the larger centres. Bunnings usually have them and any town big enough for a Bunnings will have a source for refills.

Why don't you just remove one 9kg bottle, rather than changing to 2 x 4.5kg bottles?