We might be able to quickly save you some money on upgrades here. Shotcut does not use the GPU for frame processing. It is CPU only. Money would be wasted on the 4090 unless the plan is to use hardware encoding all the time, specifically AV1.
Overall, your rig looks pretty good already. Proxy and Preview Scaling would be the next optimization step, as already mentioned by @MusicalBox.
More details on the performance effects of other components:
More details on what Proxy and Preview Scaling bring to the table: