I don't see a Trek FX Sport 9 listed anywhere I have seen the FX6 and in general the parts are decent 11 speed parts but the frame and fork use a very odd sizing for the wheels and their silly thru-skew which is not a thru axle and not a quick release which is dumb as those are the two commonly used wheel holding standards for most bikes (you also have bolt on but less common on carbon) If something happens and there is no Trek dealer around you are potentially SOL for putting that wheel back in. Plus with the Trek you are paying a little extra for their logo.
The Kentfield is a low initial cost hybrid, the AdventX is decent but it only carries through derailleur and shifter and the cranks are cheapies. The brakes are no-name and the frame and fork are average and a huge step down from the carbon Trek. Not a terrible bike but not one that excites me.
If I was going for a flat bar Marin that wasn't a custom build (we did a custom build carbon gravel flat bar frame for someone at my shop) it would be this bike:
https://www.marinbikes.com/bikes/2024-larkspur-2
Nice wide tires, comfortable and upright steel frame and fork that is easy to ride (step throughs are awesome for anyone but especially anyone with male genitalia), decent Shimano drivetrain and hydraulic brakes. Plus it has a dropper post for those times you want to go off gravel a little or have an easier time getting on and off the bike. It is a sweet bike for the price.
Or I would look at the Kona Dr. Dew which is also steel but has 12 speed Deore and some nicer parts all around.
https://www.konaworld.com/products/dr-dew-1
If you like the Carbon then the new Specialized is pretty unique but decently spec'd:
https://www.specialized.com/us/en/si...=322033-200214
Plus in stark constrast to Trek they are doing a lot more minimalistic branding so while you are paying for Specialized quality and support you aren't having to put a bunch of extra money for a logo. Not saying they aren't maybe a little higher priced than needed but not by crazy amounts and they have really excellent support so it pays off.
I would also potentially look at the Diverge EVO which is/was their sort of higher end flatbar gravel bike in aluminum and maybe they had a carbon frame in the line up when it first came out but I cannot recall.
Or if you want Marin again in Aluminum the DSX 3 would be the choice:
https://www.marinbikes.com/bikes/2024-dsx-3
However in the end the big key is going out and test riding these bikes. Find one that fits you well and you are comfortable on for the test riding blocking out the saddle, grips and pedals as those are irrelevant for a test ride and tell you nothing about the bike and at least grip and pedal wise would be replaced right away (unless they came with Ergon grips from the factory). Saddles are something I tell people to adjust and ride for some longer rides and get used to it before you get rid of it but pedals that come on bikes are cheapie test ride pedals and grips usually aren't Ergon and Ergon is the way to go.