yeah i was looking at borg31 before i decided to build my own with dt 370 track hubs, dt rr 511 rims, dt alpine iii spokes.>>1667004
mavic ellipse are probably alright but imo they're probably a bit dated to be honest, they only have 15C rim width. i would prefer a 10mm rear axle just so that the bike's geometry will be perfectly as intended. and i've read they're a pain to service because they use nonstandard spokes.
cx-ray spokes are a bit of a meme if you don't have deeper rims and you give up some stiffness compared to thicker spokes. on road bikes which have a narrower rear wheel they might get a significant amount of flex during sprints and such although it might be fine on track wheels. dt t 1800 classic wheels do use dt aerolite which is the dt swiss answer to cx-ray. there are cx-sprint and dt aero comp for a better balance between aero and stiffness. you could also go for regular double butted spokes to save a bit of money or to free up some of your budget to go toward other rims or hubs.https://novemberbicycles.com/blogs/blog/from-the-vault-spoke-aerodynamics