Is a 5800X3D and 9070 XT a worthwhile upgrade from my current PC?

I'd start with the 9070 XT in the current system and see how it performs before buying another CPU and motherboard. The 12400F is still worth testing with a faster GPU, and the amount it holds you back will depend on the games and resolution.

Check the power supply and case clearance first. If the existing PSU isn't suitable, replace it as part of the GPU upgrade. Then see whether the games you actually play are CPU limited enough to justify spending more.

The 5800X3D and 9070 XT can work well together, but buying into AM4 from scratch is the part I'd question. At the listed 549 AUD for the CPU, compare the total cost with an AM5 build using something like a Ryzen 5 7600 or 7600X.

AM5 needs DDR5, so your spare memory wouldn't transfer. On the other hand, you get a newer platform and more room for future CPU upgrades. Compare complete system prices rather than just the processor prices.

If the GPU upgrade exposes a CPU limit, an Intel upgrade on your existing motherboard may be another option. A 14600K is worth pricing, but check the exact board's CPU support, BIOS version, power delivery and cooling before buying one. It isn't automatically a drop-in upgrade on every board.

The 32 GB capacity is useful, but DDR4-2400 is on the slow side for a Ryzen gaming build. Check that it's standard desktop memory and that the motherboard supports it, especially since it came from a workstation. It may save money up front, but don't assume it will perform like a faster DDR4 kit in CPU-sensitive games.

For storage, check the NVMe drive's health and capacity before making it your main drive. Reusing it is reasonable if it meets your needs.

What resolution and games are you targeting? At 1440p, the GPU can be the main limit in many games, so a more expensive CPU won't always make a big difference. Games with lots of units or simulations can behave differently and benefit much more from CPU performance, especially if you're chasing high frame rates.