> Brexit was a remarkably straightforward process, you send a letter and at the latest two years later you're out if you don't ask for exception
Straightforward is a bit of a stretch.
Brexit was convoluted because the UK is in a weird position where it had a ton of agreements regarding its border with France which had not been negotiated for a long time as they didn't matter when they were part of the EU and they wanted things which were not compatible with the Good Friday agreement. As far as I know, this part is still not settled.