For the money, if you can’t build something stronger, wider, and lighter and less hassly you’re probably doing something wrong. Can you pull DT240s on supercomps on lightbicycle carbon in under budget? Maybe not quite, used and reconn’d 240s would be grand though.
Mavic spares are available without too much fuss btw, though you’ll always need to special order them. They’re a good company in that respect (I got spares for 7-year-old old-style Enduros) but still not cheap, and you’re tied to their stuff.
(Aside- I really don’t understand the logic of fitting a weaker rim on the back, it’s not like the SX is super tough (or overly heavy). Assuming the ERD is the same I can see a lot of people replacing knackered rears with front rims.)