IMO 1944 on XXI it is not hard.
You simply have to periscope depth dive in dock.
There are unfortunately a few bugs in the type XXI (in GWX, too) as the radar antenna or sometimes RWR broken. Also changed are the front and rear batteries when they are damaged. Also in contrast to types VII and IX, RWR appears on the snorkel not 1 October 1944, but 1 January 1945. A little weird.
But despite that swimming at XXI is very easy. Submerged with silent-run to overtake the slow convoy, and then load the torpedo while shooting. Recently, when I tested the patches for a type XXI I was able to sink almost all ships of average convoy and two other destroyers escorted by torpedo T5.

Really difficult to pull out of harbor in norway in bad weather. I've taken damage from hitting the bottom, mid channel, while surfaced in 15 m/s weather. Not very smart to travel through a port submerged either.

U-Boot Type XXI has a tendency to fall into oscillation, which was a problem similar to those that took the first torpedo (u-boot is very unstable longitudinally, at least in SH3). To avoid this, do not dive deep on the controls, but in a ballast tank at a speed of 0 kts in the dock. Only then should move. When the first attack ends (usually pass in no more than 4 km), you can usually safely surface and and pass the rest of the way through the fjords.
To avoid major damage due to hitting the bottom of the bow or stern as a result of these oscillations you can install "Seabed repairs" mod which forms on the u-boat bottom extra armored zone.

"install "Seabed repairs" mod which forms on the u-boat bottom extra armored zone."

This mod is listed for GWX 2.0-will it work with GWX 3.0?
Thanks

Yes. I am sure it will work. Files modified by this mod has not changed since GWX 2.0 so it's compatible with GWX 3.0.
I'm using this.