gears overshifting issue
Since everything else is working, this is when I suspect hanger alignment. Where the rear derailleur bolts on to the bike, just under the quick release/axle, is a relatively fragile piece of metal. They get bent if the derailleur is bumped hard, and for that reason are often replaceable parts.

The problem is sufficiently common that there's a tool made specifically to fix it:



Those are about $AUS40 to buy, but you're not going to use it very often. Probably once. But your local bike shop will have one, and it's probably less than the cost of the tool to get them to look at the problem. Or you can buy the tool and look up how to use it (it's not hard). Without the tool you're very unlikely to improve things by trying to fake it.