safest bet imo for stability is probably the ASRock Z790 Taichi Lite. it has a massive 24+1+2 phase vrm setup which is plenty to keep a 14900k stable during those long 8k renders. video editing puts a lot of stress on the power delivery for hours at a time so you dont want a board that gets too hot and throttles. it has built in dual thunderbolt 4 ports too which matches your requirements for fast external storage. i like that it skips the expensive rgb plastic shrouds that the premium boards have so you get flagship performance but at a more reasonable price point. honestly for a workstation you just want it to work and this one feels like a tank. just make sure your psu can handle the draw when both that 4090 and the cpu are pinned. its basically the full flagship board but cheaper cause it looks plain.

ROG Maximus Z790 Dark Hero: https://www.amazon.com/ASUS-ROG-Z790-Motherboard-Thunderbolt/dp/B0CJMT2723/?------123456890?6890-20

Mid-tier motherboards in the $-$350 range offer good value. Asus and MSI are reliable brands.

The RTX 4090 provides excellent performance per dollar, but its necessity depends on your work. For 3D rendering, it's ideal. However, for After Effects or Premiere Pro, investing in 128GB RAM might be more beneficial.