The last round ofFormula Foil Racing was held at Kurnell in a cross-onshore N/E with rising swell upto half a metre mostly. So not impossible to navigate, but of the dozen competitors, only the top 2 or 3 made all 5 races 'dry'.
When it gets over 15kts I've been switching to Slalom Foiling or Fin-ing so haven't really practiced in the upper end of the wind spectrum for racing gear with big sails and big foils.

Upwind
I find sailing upwind in swell relatively easy. Slower board speeds and power more in control make it easier. Some big bumps I have tapped the top to make sure I don't come out the other side, but mostly in control upwind. Especially in swell below .75 to 1m.

Downwind
This is where most of the drama was had. For those that don't know, Kurnell has an opening to the sea with some big swells coming in from the ocean. These combine with wind blown chop. When sailing downwind you can bounce over 2 or 3 bumps then the 4th one seems to have excess power and lifts you harder than previously. Once I was ready for this but the opposite happened and the fatter looking swelll didn't push as hard and I ended up dropping instead!

How do you ride swell lines fast downwind?

I don't.
But I talk to 3/4 of the local Cal Cup fleet, mostly on 105 masts, a handful on 95.
They say..stay fluid, look around, never head uphill, be ready to adjust direction and attitude, be ready to tilt the board slightly, run appropriate length lines for the power and your direction, pump when needed, and a few other things that I shut my brain to, since I plan to never course slalom again.
Obviously, most racers don't share their secrets. But if you're around all the time, you can catch even the most secretive racer at a vulnerable moment.

We're going to find out when they start the IQ Foil test events for the next Olympics in Marseille. The short video I saw had a ferocious confused sea state with wild winds. The sailing was exciting. Garda has monumental chop though no swell & the fleet seem to manage it okay.
Next time we're out, we can get some downwind tips from Jose! www.sailing.org.au/news/nsws-first-foil-lab/

IQ guys raced a Cal Cup at Berkeley last year, after the Gorge events.
Wind 20-25 mph, high tide, out in the Olympic Circle, they mostly finished mid pack on their 9's.
Lots of crashes for 9's or 7's.
They did well. Much better than before the Gorge Races.