yes you may buy that 15 m Soul to fit together with your 9 m Speed5, tried both and will work fine.
Foils are great as travel quiver as they have both, wide windrange and being light, additionally no Need of a pump or the Risk to have damaged bladder, etc.
For myself as ultimative light and wide range I would calculate one Sonic2 6 m² or a Peak4 in 3 and 8 m² both will have around 1,7 kg so together with a foil and all stuff would be possible to stay within 10 kg for a 7-33 knt range … so if I will be succesful with a two part strut construction it would be even hand luggage compatible
Also heard a view things about Slingshot/Phantom (Elf kite canopy, great construction for experienced guys going for full race only).
I think if you are well experienced and enthusiastic to set up bridles by your own, not by attatching the 4 cook proofed bar lines attatchments, but by finding a good trimm by mm for every of the around 80 attatchment points on the Elf canopy … this kite could be a just great project for you. If you are even a bit more masochistic, also a pansh kite could be a good choice.
Knowing this first hand, as I have done this myself with first available kites 20 years ago and also know very competent friends from competition used Elf …
But if you are a normal average kiter, who just want to kite to have fun, you better go for a Soul or something similar, which can be also cheaper as a used Speed4 or 5. But this is not what this Topic was About, by the way.