if you want to do as much dh as possible, but not on offical DH runs, then I think that gives you only a few serious options….
….ie somewhere with great lift access (and by that I mean SKI lifts, not vans, because a ski lift will get you back up the mountain at least 3 or 4 times more quickly than a van ever could…)
…but away from the main DH resorts (if you prefer natural singletrack to man made stuff)
….and therefore ideally I would say guiding would be worth it very much (because the best single track is usually unmarked and hard to find, and if you buy a package, it generally works out as extremely good value anyway)
All that in mind I'd wholeheartedly agree with previous recommendations on this thread of Verbier or Les Arcs.
By the way, Whiteroom are a) not in les arcs (but rather in st Foye, about a 45 minute van ride from les arcs centre) and b) there are no ski lifts running in the summer in ste foye (rather, they have a van going up and down the hill) although I have seen from this forum that they do come well recommended so they must be doing something right, I guess it depends what you are looking for.
If you do a quick search, you'll find alternatives for Les Arcs that are also highly recommended on this forum! 😉 For Verbier, I understand that the gold-standard in terms of service, is BIkeVerbier.