This.
The problem you've got OP is that you don't actually have the luxury of time to decide or to "try longer/harder to adjust". It's decision time now.
You currently have two children, neither born there, only been there one year and one of them is unhappy and wants to return to UK. You and he has an agreement to try for a year, now the year is up. You spoke to your H and the agreement you came to was that you'd leave with DC and he'd visit UK often. So as it stands you can return to the UK. In the current circumstances I think it would be hard to argue the DC live in Australia now.
Your H has told you in several different ways that he has no intention of returning. He's literally told you with those words, although he said "probably" it's clear his mind is made up 100%. He's signed for a 4yr training course. That's not the actions of someone who doesn't want to stay long term. Even the prospect of losing you and DC hasn't made him reconsider for even a minute, his response was "ok then you go".
I would not discussing it any further with him and giving him the chance to withdraw consent for DC to leave. If he wants to argue the toss about how he's changed his mind or didn't really mean it or didn't realise you'd follow through, far better for you if he does that after the event when DC are on British soil. Than dragging out any argument while they're still in Australia and all that time adding to his argument that their lives are there now. Once you're back in UK surely he'd then be the one in the position of needing your consent to bring DC back to Australia to live and you could refuse to give it.
If you stay another year, it's going to look more like the DC lives are in Australia and when your H realises that he won't be a single parent if you two split up, because all he has to do to keep you there is to refuse to let DC leave (knowing you won't leave without them), that's what he'll no doubt do. You think your marriage will survive him doing that to you and controlling you in that way? I don't.
I can see you ending up a single parent either way. With all that entails. The only real choice I see is whether you're going to do it in Australia or UK.
If you're close to your family no amount of time is going to make you not miss them. If you don't like the heat, are you going to somehow change that personal preference? UK athletes training in foreign countries to acclimatise to the heat and humidity prior to the Olympics don't have to go there for years to do that, more like a few months at most for their bodies to adjust. I think the people saying you just need to get out of the house more have missed the part where you said you stay home a lot to escape the heat. The issue isn't that you haven't made friends and are lonely, the issue is that you miss your family and don't like the heat. I'm going to presume that since you have friends and eyes in your head, you're already doing whatever it is that Australians do to combat the heat when outdoors and for you those measures are not enough to make life enjoyable.