By the way, it was the border with Ireland that created issues, not the one with France
It was an admittedly "um actually"-adjacent comment, but I think still justified because Switzerland would have the exact same issues if they just renounced their treaties with the EU so they'd need to undergo as complex a negotiation as the UK had to
As a French, let me tell you there were and still are plenty of issues with France too notably regarding fishing rights and border control. France didn't threaten to turn off electricity to some of the Channel islands because everything was fine.
The UK could choose to leave and cut off all agreements concerning fishing rights, the tunnel, the border checks with france and it'd have been perfectly legal, if diplomatically damaging
With the Irish border there was/is the complication of the GFA, which prevented the same kind of approach
> 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.