You would think this to be a simple matter, easily settled. But no..
Australia has a TA of 10,000 feet and a TL of 11,000 feet then says nobody can cruise in the layer. So if you are going West you cannot use 10,000 and are forced to go down to 8,000 (IFR) which could and often is affected by weather and ice. And for some reason Australian pilots are unable to function without oxygen above 10,000 so they cannot fly at 12,000, like American pilots can do, so there is only one altitude available in a lot of the Eastern states due to the high ground around the Snowy Mountains. (if you fly unpressurised of course).
NZ uses 13,000 feet, and that would be perfect for Aus, but no, it was notinventedhere. Then NZ has high mountains in the South and a TA of 18,000 would work better there, but no, notinventedhere.
Even in the US, the TA of 18,000 does not work in Alaska, where mountains go up to 21,000 feet, but I suppose a TA of 22,000 would be difficult to manage. As a photo pilot, I have a lot of trouble with 18,000 in Alaska, it makes a lot of my flying IFR and since the US does not use photo blocks (another piece of notinventedhere) some areas are extremely difficult to shoot.
Now along comes Europe, and is bound and determined to reinvent this wheel. The system of varying TAs is stupid, always was and always will be. But please, don't fall into the trap the Aussies set for themselves, and don't dismiss 18,000 just because the cousins use it. As in most things aviation, simplicity is good.