Fit OEM ECU -> do adaptation -> fit your standalone.
Your standalone ECU can not process the requests from the gearbox for increasing or decreasing RPM, can not send accurate calculated torque, both of which are absolutely crucial for completing the adaptation in the first place.
The adaptations have actually nothing to do with the engine, so if you can adapt it in a different car, that works too.

Adapting it with a standalone is never going to work. Driving and gearshifts won't be also too great, but doable if car is for racing use.

Yes, this is the main thing you have here "supposed to".
In reality this is not true, the bare minimum is done.

The gearbox needs to be able to change the engine speed setpoint to complete adaptation, it can't do that, thus the adaptation can't complete.
Also "drives great" is very relative. Having some experience with some of these matters - you can never make the DSG shift and behave correctly, when your entire torque calculation is a single small map based on e.g. MAP and RPM or TPS and RPM (like it is standard).
In fact, I'd dare say, do not under any circumstances adapt the gearbox, you will probably make it drive worse, than if it was ever adapted on a standard ECU.

There is no way to make it drive "great" without having at least a representation of temperature corrected friction torque of the engine, never mind IGA efficiency.
All the cold start and very hot weather stuff is way off, and because the firmware of the ECU is so limited, there is nothing you can do about it either.

You will also see significantly accelerated wear on the clutch pack, but this is probably not too important on a project car, as you won't be doing a lot of mileage.

Pressure on clutch is closed loop controlled by microslip adjuster.
Unless someone has turned off the microslip adjuster by using a hack.
Re-adapting the gearbox will not change the pressure, it will be always exactly the correct pressure with some initial onset over-/undershoot(unless hitting certain limits, in which case still, adaptation won't do anything.

Correction, after a hack where microslip correction is turned off and the adaptation routine hacked it is neccessary to do that.
After a proper remap where the gearbox isn't messed up it isn't, but there are very few people who know how to do that.