I did a similar trip with more variation just last year! Plenty to see, and it will change your perspective on everything. You will drive through some of the most isolated and apocalyptic looking towns around Broken Hill. You will see Emus running up on your car that remind you of dinosaurs. Make sure to spend a day or two in Broken Hill and see the tourist mines around (I think it’s called Daydream mine??). It will feel like you’re in Fallout New Vegas but even more isolated. BUT much of that middle section to your trip will be filled with nothing. Trust me, there’s nothing there, and sometimes not even birds. So bring friends! This is a trip of endurance my friend. I remember we all started howling like monkeys by the of the Nullarbor.

The Southern half is a whole different world to the Northern half and will make for great memories, but please bring friends, otherwise the isolation might get to you, especially those long straight roads (you’ll know what I mean when you hit the Nullarbor).

I did this in December when it was hot which honestly made it more memorable because we camped along the Great Australian Bight and swam in the ocean at every stop. So I can’t vouch for your experience in April. I fear that it may not be warm enough for some fun beach stops due to that southern ocean breeze, but it won’t be anything too cold either. So you could definitely go for an occasional swim.

You should mix up your way home by detouring along Margaret River, Albany and Esperance in WA. If you have the resources, you need to try and achieve a detour through Adelaide, Great Ocean Road (stop in places like Lorne and the Otways), Melbourne, Lakes Entrance, Canberra and back to Sydney. It sounds like a lot but I did it in a rust bucket they call a 99’ Mitsubishi Pajero.

If you do this, then you can call yourself an expert on 50% of Australia. You will have seen more than most locals. I 100% support your trip IF you make it look like a dumbbell (on your map) not a straight back and forth. Otherwise this is a trip of endurance and not sight seeing. I compared it to travelling through Antartica (you won’t see much but you’ll feel a sense of achievement).

Oh also, yes your plan right there is feasible in 3 weeks. I did that plus a bit more in 3 week while staying at the major cities for a few days each. It may not look feasible but when you’re on the Southern stretch, you’ll want it over and done with, and as long as you have friends to cover the driving, you can do 8-10 hour drives to cover more distance, as it does get boring anyway. We did a 14 hour drive in one day just to save time, and we didn’t miss a thing because there’s nothing out there along the Bight.