Sauna install

Hello everyone, first post, not an electrician, so hopefully this makes sense.

I have a two-person sauna from Costco that I built with a floor on casters to move against the wall and back when needed, which is why I would like to have a plug(on the room wall outside sauna) and not hardwired.

In the instructions, it says the conduit/wiring needs to be rated at 90* C with flex conduit. I have only noticed an 80*C Dry/60*C wet conduit at the local hardware stores. I have also read that under the sauna is significantly colder, and the conduit will only be about 4-6 inches inside the sauna before exiting.

The plan is to remove one of the 240 dedicated circuit (removed an on-suite kitchen and have 1 in the garage that I don't use) and place a 30amp possibly gfci breaker and run new 10 gauge wiring to an outlet near where the sauna is placed.

Questions
Do you think the 90* ref is to just the wire? Do they make a higher temp liquid tight flex conduit that's readily available?

Can I utilize for ease of installation a 10ga 3 prong dryer cord to run through the conduit into the heater for the Line 1, Line 2, and ground? I cant seem to find if dryer cords are temp rated (NM-B).

Running new wire to the breaker panel, should I run a neutral and cap it off in the outlet box or just run 2 hots and a ground(10/2)? It's not to a dryer technically...or in a room where you think one would be at.

Lots of threads on the GFCI debate on a sauna on the internet. Even found an older manual of this model online and said not too. Still trying to research that and reached out to the manufacture why they removed from this manual

Thanks in advance for the help!