Car appreciation... the aesthetics, the engineering, etc

Is it worth just doing all four at the same time from a driving/safety perspective? The 4th has a couple of mm more than the 3rd, but it's not like it's significantly newer.

No, if the tyres are safe and legal there’s no need to change all four at the same time, unless it suits you financially / time wise to do so. You already have tyres with vastly different thread depths (hence changing two) and you haven’t been ditch finding .