Imo for Australia I think that will be a more realistic long range strike platform than the B21.
Remembering B21 is subsonic, won't have an antishipping focus for the USAF, won't have a radar, won't be "multirole". Against a peer like China who will have AEW, long range ground based radar at various frequencies, networked forces, highly capable data linked antiair missiles, I'm not sure "Stealth Bombers" are the future for that space. Its not a replacement for the F-111. Focus for integration will be nuclear weapons, gravity bombs, glide bombs, not missiles like LRASM. Not unless you have degraded their capability first. The US can do that (to some nations, I think they would struggle against China, but US has mega capabilities).
But Australia won't be knocking out China's mainland defense facilities, and if not China, then we are causing issues with our region on why we would want this kind of bomber.
The P8 is also a useful platform (ASW, ASuW, search and rescue) and is the US maritime strike air platform. It also is low to operate on showing the flag or freedom navigation flights. For Australia we could give any hostile remote isolated forward operating base a bloody nose with P8 long range strike. Its credible.
I don't think the B21 will be low cost anything. I'm not sure how survivable it would be into the 2030s against the kind of peers we would want it capable of.
A potential hick-up is P8 integration with AGM-158B2.. this USN plane can't fire USAF weapons.. Again, even though it would be really handy to do so (but might hurt b21 if it could). Might be worth pushing the US on this integration perhaps with the UK, India, SK, etc. I know JASSM-ER and LRASM are highly similar, but that isn't the same as being a proven firing platform and fully integrated.