Would a 5800X3D help my WoW raid FPS over a 5900X?

The 5800X3D is a reasonable upgrade to consider if WoW raids are your main priority. Its larger cache can help in CPU-heavy games, and busy raids can put a lot of pressure on the CPU even with a strong graphics card.

That said, the size of the gain in your particular raid is hard to predict. Check the price against how much those low frame rates bother you rather than assuming it will fix every drop.

I would troubleshoot before buying anything. A 5900X is still a capable CPU, and 25 FPS with reduced settings is worth investigating. Try comparing DirectX 11 and 12, check whether background programs are using significant resources, and see whether changing settings makes a noticeable difference in the same kind of raid situation.

If lowering graphics settings barely changes FPS while the GPU is not working near its limit, that points more toward a CPU or another system bottleneck. It does not tell you exactly how much a new CPU would help, though.

Don't dismiss the problem just because WoW is an old game. A crowded raid with players, spell effects, and encounter mechanics can be much more demanding than quieter areas. Performance in a less busy part of the game won't necessarily tell you what to expect during a fight.