Looks like Mission Tuning website was updated now and you can purchase the tune for the M340i now. Unfortunately - it sounds like for all cars with the latest DME update (from July 2019), you will need to send them your DME so they can flash it. So that means being without your car for at least some amount of time I guess? Unless you live physically close to their location.
I remember back in the day when Hondata did this on RSX Type S's they offered an ECU swap - you put down a deposit for a flashed ECU that they mail you, and then you mail them back your stock ECU and they refund the deposit. So you have virtually no downtime. Not sure if BMW's DME is tied to something else in the car that would make this impossible - but it'd be nice if they had something like that - or a way to buy an additional DME unit.
https://missiontuning.com/bmw-engine...lash-tune.html
For cars with the July 2019 update, I also assume you can't go back to stock yourself, so then this is probably detectable by the dealer and for a leased car if you want to go back to stock, you'd have to mail the DME to them again? Unless of course they figure out a way to flash it locally in the next 3 years. Not really ideal for most of us leasing though.