Good to hear you are ok Tom
Yes, you should have been more careful winding your lines up, definitely.
Sit relaxed on your board (and practice this as you had difficulties doing it you wrote - but is easy) winding the lines up FULLY, while you think about the situation and what to do.
I have heard, that letting your board go, and resting on the inside of your kite with your upper body free just over the LE so you can swim with both arms, and got good flotation, is key to "survival", but havent needed it myself yet.
Oh, while writing, I can see edt has written the same.
We have had many really long swims around here too, also a couple of medium ones in the last few months where water is even colder 1-2 degrees (same water as yours just on the other side of Kattegat)
I also avoid riding far out when the water is like that, as anything can happen - but sometimes we still get "Caught" in something, like last week out in a perfect steady quite onshore breeze maybe 9-10 knots so lots of wind for 10 m2 and bigger kites.
Suddenly the wind backed off in a split second, my kite fell from the sky during a jibe, coming with normal speed in, losing power in the carve, and after that the kite simply backed down even when I pulled ONLY the frontlines (where you can fly a kite in 1-3 knots of wind).
I was close to shore maybe less than 100 m, but it took a looong time to get ashore as the wind was gone down to ZERO !
If there had been any wind at all left, a few knots or more, I would sit on my (extremely small but does not matter, it works have done it often) board, and point the board/mast towards shore, while pulling the inside rearline so the kite also turns towards shore (if not dead onshore marginal wind).
So close to shore I kept my bar in the loop (or the safety leash in suicide), put one arm around my board and swam with the other arm - sometimes turning around to use the other arm.
Takes foreeeever, especially because when no wind at all, your kite will drag after you, instead of in front of you like normally if a tad of wind, and be an anchor especially if strutless so you get almost nowhere.
I should of course just have turned my hook around on my back and paddled with both arms, me belly down on my small board - works awesome, but I was so close to shore.
Two other guys with foil kites got caught too, one got almost ashore and could reach the bottom before the kite dropped - the other had to pack his foilkite down, and swim for several hundred meters ashore with his kite on his board - used to it often but still quite cold water.
No issues here though.
Had an awesome day/session that ended suddenly with 3 swims and walking for the other guys, with really good "perfect" wind earlier, so it did not matter for us it ended like that
The fastest way to get ashore, is to ditch the kite and swim on your board IMO, even if it is a thin small "plank" board - it still gives you flotation and you will swim faster
But you have to be sure you can get ashore, as now you have no "blimp" and can not be spotted easily nomore, so risky to ditch your major flotation device.
Can be used in a bit more closed bays when wind drops to zero, and you can retrieve your kite later somehow - easier when you are ashore and out of the cold water
Some bring their mobile phone in an Aquapack thing inside the suit, so they can call for help if needed, as when out alone or if noone have spotted you, it is possible to call for help, or use it when ashore.
Has been used several times, also when one got ashore FAR far away when the wind died, and he could call when on land, for a pickup to get home to the launch spot.
But 7-800 meters is very far in cold water Tom, indeed.
If a bit warmer, you can sit on your board let the kite pull you slowly, and enjoy the other riders flying around you if some out with bigger kites, and maybe even wait for the wind to pick up again, or a gust, so you might get lucky and can relaunch.