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Do Shimano MTB hubs have sealed cartridge bearings?
No. But their sealing is actually very good. If you are careful with them, they'll outlast the sealed bearings in most hubs by some margin.
Obviously if you aren't careful with them then you'll toast them and need to replace the entire hub.
Novatecs are nice, but some of their hubs use sealed bearings of an unusual size, making them very pricey to replace, relative to the cost of a new hub.
FWIW Superstar Switch hubs are rebranded Novatecs. So a fair choice if you don't mind their branding and a 32H build.
You need QR and I'd go for 6 bolt rather than centre lock.
Do Shimano MTB hubs have sealed cartridge bearings? If not then I'd get the Novatecs. Some of their range can be switched between QR and thru axle which will future proof you to an extent.
I've been running two wheel sets with Novatec hubs for 18 months and have had no hub related issues, despite plenty of abuse.