So racing is the best workout to get better at racing. The pros use lower importance races to get fit for their target races (and so should amateurs - if you have enough races on your schedule to do it).
CX is similar to a threshold workout in that you stay working at a hard and steady effort, it just has lots of high spikes rather than a TT where youre trying to sit on a wattage. You would prob get burned out working threshold workouts midweek then race CX on the weekend. I would just do some 30/30 midweek and race CX on weekends. The actual CX race like you said is a constant high HR event where you do sprint for position in the first lap but otherwise you’re trying to be even paced (per lap - not like an even power output on each lap). I put the power required for the given terrain (high torque stuff) but otherwise sit on SS.
If you’re ftp is actually going down thru CX season, it’s more bc the actual event isn’t training it bc it’s not important for the actual event. Tho, I think it would stay relatively constant throughout the season.