Mellowing out a KX450?

Have you tried the KX ignition couplers? Aren't those in the mellow, revvy, and gnarly flavor?

Try 1 up on the countersprocket to see if you like where that goes. Usually does the trick for me.

Also, don't be afraid to put some slack in the throttle cable. As I get tired throughout the day and I'm less precise with the throttle, I add slack (easy on YZFs, hand adjustment when pulled over).

Finally, my favorite "nuclear options" = Rekluse Radius CX + Vortex ECU programmed by XPR. Amazing products, Rekluse takes some 450 edge off, just enough. XPR gives you great options to try.

If you decide to sell that bike, please look me up.

I bought my 2020 450 sxf used, already had 1 tooth bigger on the countershaft.  Initially i thought i should go back to stock but i love it, mellow power.  I'm 37 vet rider..  My 2017 fx350 seems snappier even, but it's all in the gearing.  

Only downside is idling thru pits, the taller gearing makes idling in 1st gear kinda fast

Black coupler or remap with the Chavez map is a good place to start. 13-50 gearing comes stock, but with a 114L chain. Throwing on a 116L lengthens the wheelbase and helps the bike drive better, in my opinion. Pair that 116L with a steel rear sprocket and X/O-Ring chain and the abruptness will mellow out as well. Can always go 14T counter and experiment with 50/51/52 rears, but I have no experience where it will put the rear axle or how the 450SR will react. Cheaper and easier to test gearing (as well as that throttle tube you already purchased) over other options on any modern 450.

Best of luck and keep us updated!

14/52 gearing

If you move the axle far back on the KX450, they are prone to bending the chain adjuster bolts, especially for aggressive riders.  The bolt face does not get a nice solid engagement on the face of the axle block.  It's kind of half-offset.  Really non-ideal.   I was helping a fast guy that kept bending the left bolt.  Best solution I found was the Pro Circuit blocks and stronger aftermarket bolts.  The PC blocks have a nice long nose on them so there's a lot less thread exposed.

Does a flywheel weight make sense in this situation?

You can get a less aggressive throttle cam from Jeff Emig

You can also put a smaller end pipe in the silencer. We use acrapovic and there its really easy to put in a smaller one. PC should have a spark arrestor or something you can buy that fits yours that you can modify if needed. Go smaller until you get it as tame as you like. Will give it less of "snap" and be more quiet as a bonus. Next step is to lower the compression (dual base gaskets). Those mods with what you allready did should make it really nice and smoth. 

A flywheel weight is a bolt on weight that goes on the flywheel.  Typically very easy install and removal.  I don't know if that's a true statement for your bike though.

The flywheel is connected to the crank.  You add mass to the crank, it gives it more inertia.  They are most commonly used on woods bikes (at least that's how I know of them), where the added inertia helps prevent stalling by keeping the flywheel rotating and smooths out the sudden hit and makes the engine rev more slowly. I have used them only on 2 strokes, which have a sudden increase in power as the bike revs up.  These can all be advantages in the woods but are not characteristics people are often looking to add to their MX bike.

https://www.steahlyoffroad.com/kawasaki-kx-450f-xc-19-21-flywheel-weigh…

Looks very simple on the KX450.  Direct bolt on.  Don't even have to pull the flywheel.

Similar to reducing the exhaust pipe hole you can close up the air box. Keep the back fire screen, or through on a second "skin"  on the air filter. 

A stock kx450 exhaust, the black coupler and maybe a 48 tooth rear sprocket or a 14 front sprocket would do the trick.

If u dont wanna go the stock exhaust route you could just reduce the size of the end cap of your silencer as someone allready mentioned but generelly speaking the long stock silencer will give you smooth power from the bottom. The shorter silencers will often give you a little more hit. 

Black coupler and 49 or 48 tooth rear. Stock pipe or an Fmf 4.1 makes it smoother. 
 

pc pipes hit hard in my experience. 

Pull some timing and add some fuel in the throttle position and rpm range you are having problems. It will slow down.