I would see if I could find some trainer software that allows you to 'ride the course' in some kind of simulation mode that you can visualize
I would do some math on what I believe my gearing/weight/power would be and probably do some 'big gear' work.
I would figure out when I will be coasting and with climbing do some simulation rides to zero in on my pacing.
I would schedule some type at a training camp or elsewhere to practice descending.

<pink>I would follow Jay's advice and practice my shopping as well as the other chap and learn to order breakfast in French </pink>

1. Bottom line is, that it's a long distance triathlon. Volume & intensity has to account for it.
2. It's a difficult course, mostly because of the hilly (factor 1) and relentless (factor 2) bike course. It's important to have the legs to climb, therefore I'd recommend to include high gear / low cadence work on your trainer. The climbs are 20-60 minutes long, where you'll be be going higher than zone 2 - account for this. On the plus side, you don't need to worry about staying aero all the time - the course naturally gives you chances to sit up.
3. There's a high probability, that the water conditions will be calm. They usually are calm in the morning. Our AG race was 50/50, because the conditions changed mid-race. But nothing like IM Ireland. More than ocean swim I recommend to practise mass start swim. It was a total clusterfuck with elbows and heels landing on my face for the first straight line (850m). Don't be afraid to join packed swimming pool lanes.

Is the world championship a mass start for age groupers? I've never had that experience before. Used to have terrible swim anxiety, but I seem to somehow have mitigated it with a couple IMs under my belt now. The ocean swim freaks me out a little though, especially with no wetsuit...but buoyancy + salt water will help I suppose.

It’s a mass start for every AG. E.g. in my M30-34 there were around 230-250 men starting at the same signal. 5 minutes later there was another group of ca. 200 men etc.

Wetsuit / no wetsuit: we were borderline with 24.8 Celsius. This year has been extraordinary hot, normally it’s 22-23 Celsius water in Nice at this time of year. Also, I suspect the 2024 Nice WC will be 1-2 weeks later. That was the original plan for this year, but wasn’t possible due to the Rugby WC in France.

A mass start for every age group...sounds like quite an experience! I'm in the 35-39 AG. Nice next year is September 22nd, so possibly wetsuit legal it sounds like. A mass start sounds a bit intimidating, but yolo.

I did Nice 70.3 worlds in 2019. Those types of European mountainous, winding and twisting roads are hard to find in the US. I live in a VERY hilly state where it's almost impossible to find flat terrain. But it's still very different than what you'll find in Nice. If you do Zwift, Fulgaz or other similar indoor applications I would ride some hilly courses at least once a week. Not the same but better than nothing. I'm sure you'll finish and do fine but adjust your expectations since you don't live in Europe. It's the descents that are the challenge. Those folks able to train in France near or on the course will have the advantage. I will be doing Kona soon and it's almost impossible to acclimate to the heat here in my colder northern state in the Northeast. But I do what I can including sitting in my sauna. Not the same but I'm adjusting my expectations since I don't live in a hot humid climate. Enjoy Nice! It's a fabulous race venue. I loved it. I didn't love the 20 miles worth of twisting descents but still grateful I got to race my bike in France.

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Death is easy....peaceful. Life is harder.

Get there early and ride the course….. the trainer will be adequate for training the climbs.

Being comfortable with the descents will make a massive time difference and also enable you to take nutrition on in the last hour or so. Obviously it’s also much safer……

You can also swim in the ocean a few times which sounds like it will be invaluable.