So I have been thinking about getting a new switch and physical mario odyssey for 1.0. But in Smallant’s guide to getting 1.0, it says you must have an SMO cartridge with the serial number ending in 000. Does that mean the new odyssey cartridges end in something like 002 and have 1.2 on the cartridge? Because I don’t want to waste money on physical SMO for nothing.

Yes, newer ones will almost certainly end in 001 (with 1.1 on the cartridge) or 002 (1.2 on the cartridge). Depending on where you buy the physical copy, you might be able to ask to look at the serial number before purchasing it.

You can probably go to a gamestop and if you're nice and friendly with the employee(s) there, they might be able to allow you to check for the ending of the serial number on an used copy before buying.