I do interior photography occasionally. I'd like to replace my Canon M50 with the iPhone 13 Pro Max but I'm concerned about an external flash.

You should be. The rolling electronic shutters in most phone cameras eliminates using flash at all, since it lowers the camera's flash sync speed down to roughly 1/30s or slower (on my iPhone8, it was 1/25s). It's why the "flash" on most phone cameras is really a continuous LED light, rather than a Xenon bulb strobe.

That LED 'flash' also cannot trip "dumb" optical slaves.

Is it possible to use an external flash with iPhone?

It is, but at this time, the only company that's cracked the problem of syncing an off-camera radio-triggered flash with an iPhone camera is Profoto. Which costs a kidney (e.g., the Profoto A10 speedlight is US$1100). But the Protofo Camera app for iOS and Android lets you use Profoto lights with your iPhone camera via their AirX technology at any shutter speed.

AFAIK, at the time of writing this (Sept. 2021), nobody else has managed to correctly sync a strobe with a smartphone camera.

There was the Godox A1 transmitter/flash which communicated with the phone via Bluetooth, but it stopped working after iOS 13 and the iPhone 11 came out, and has since been discontinued. It had a very narrow window of usage, anyway, because, unlike Profoto, they never found a way around the sync speed issue, and the Godox Photos app itself (which was what you had to use as your camera app) was badly designed and wonky. I detailed using one in this 2017 dpreview post.

What's the best external flash dedicated?

There is no such thing as a dedicated flash for iPhone, since the iPhone has no physical flash sync connectors on it. And all the "flash" units that can be connected via a lightning port are really continuous LED lights.

I already own a great Canon Speedlite 430EX II. Can I use it with the iPhone?

No. Profoto does not make add-on receivers for speedlights for the Air system. And the only wireless protocol the 430EX II "speaks" is the Canon "smart" optical one.

My advice would be to consider using continuous video (LED) lighting instead of strobes with your iPhone, or if flash photography is vital to what you need to accomplish, that you not move entirely to an iPhone, but also keep a camera with a flash hotshoe in the bag.