What is the best watch for my first Chronograph?

BTW, I love that Stowa!

Sinn offers plenty of chrono choices in terms of # of registers, cases, bezels or not, additional complications, etc

I'd say to look into Damasko and Sinn! 

For sub $6400 there are many options made available to you

Maybe can consider this at secondary market 

If I was looking in that price range I'd definitely consider the Fortis Stratoliner

For that price I would definitely go Breitling or Omega, but if you like German watches, check this one out, it's a 1.5K/2K watch, way below your budget, but it's gorgeous: Junghans Meister Telemeter

Since you like German watches, how about a Junghans Max Bill Chronoscope? I absolutely love the Bauhaus design:

I love this, great shout!

I find watches in this category to be very divisive, but I really like the simplicity of the Max Bill :)

I own a Stowa Flieger Chrono Manual Wind and I love it.  You have the choice of an automatic version if you want as well.  Try to locate one used and save some cabbage.

Chronographs are my favorite complication, and I currently have 11 in my collection.  Of those, my three favorites are these (in no particular order):

Norqain Adventure Sport Chrono
The design of this one really appeals to me, although it is a bit on the large side. It's a very well built watch with a 7753 based movement, and although it's not a COSC certified chronometer, mine is currently running well within COSC specs, at less than 2SPD.  At MSRP it's probably a bit over priced, but with a suitable discount, I feel its worthy of consideration. You could probably pick one up for half of your budget.

Omega Speedmaster Racing
I tend to prefer the look of a two sub-dial chronograph over the 3 sub-dial versions, but most with 2 sub-dials don't allow for timing anything over 30 or 45 minutes. The various Omega coaxial offerings with the 2 sub-dial layout include both chrono hours and minutes in the same sub-dial, allowing for the 2 sub-dial appearance with the functionality usually only available with three sub-dials. 
At MSRP this would be beyond your stated budget, but a gently used pre-owned example should be obtainable. I know you mentioned that you didn't like the Speedy, but this is so much different that the Speedy Pro Moonwatch.

Sinn 144
This one appeals to me mostly for the aesthetics. It looks very much like a purpose built tool watch with precision German engineering, even though it is based on the Swiss 7750 movement. I like the fact that the bezel includes both a pulsometer and tachymeter scale. It is easily the most legible of my chronographs, based on the sharp contrast between the hands and dial, as well as the fact that the sub-dials blend into the dial. I also find that having both day and date is a handy feature, and I personally like the way that it's implemented in this design. It's also very attractively priced.
 

Best of luck with your decision. There are lots of great options available, and these are three that I'd suggest exploring based on my own personal experience with each.

Hamilton.

Laco, Dufa and Baltic could be brands to check out too. 

How have you found this watch? There are no video reviews of the chrono available on YouTube or elsewhere that I have found, and I'd love to hear anything you have to say about it!

Also very few second hand models that I can see, but I don't mind paying for this one new, especially with the cost of servicing a chrono

Longines Bigeye

I bought mine from a WUS (watch you seek).  See attached review of the automatic version....

The manual version is basically the same except Stowa modifies the automatic, essentially a rotor delete and add like a bridge or something like that.

https://wornandwound.com/watch-lust-stowa-flieger-chrono/

If you want something unique, and definitely worth the asking price, you can pick this up:

Sothis Spirit of the Moon Ikarus Limited Edition Made in... for $3,252 for sale from a Trusted Seller on Chrono24

It's an ETA movement, but completely redone, and decorated to a very high standard. 

This budget could get you into a Zenith El Primero, possibly new, maybe used but mint.  (I’m not an expert in USD pricing).  

1. Nivada Grenchen chronomaster manual wind or longines heritage monopusher chronograph... also Hanhart or Graham Chronofighter

Graham Chronofighter

Breitling Avenger titanium chronograph

For a first watch of any type I would not recommend to go beyond £1000. Just experiment with cheaper models to see what style/functionality you like. With mechanical chronos there is the additional issue of servicing them - which is on the more expensive side of watch service.

So, I would go for a mecaquartz chrono. The depicted watch is one I am currently contemplating. It is around £`150 and uses a VK64 from Seiko.