Hello Oldynewby,
The improvements you have laid out for your current hardware all make sense to give a good (sob, howl, sniff) bottom end performance PC for FSX. Each iteration of Microsoft Flight Simulator requires hardware at a level which increases exponentially.
However, ........ ,
One hardware improvement/replacement solves a lot of performance issues with FSX.
A, or is it ,.an, SSHD. A Solid State Hard Drive. No more waiting for a mechanical Hard Disk to spool up to 7,200 rpm and trying to carry some of the load the Graphics Card inbuilt memory (RAM really) is carrying or strike a balance between RAM (Motherboard) and whatever brand/type of graphics card you have. With Graphics Cards the inbuilt memory is more precious than gold or diamonds.
My PC started as a fairly "ho hum" tower case with the usual things. Over time the set up looks like this:
AsRock FM2A55M VG3 dual core motherboard (AMD chip 3.2 MHZ dual core.)
RAM 8GB.
Graphics card, Intel GeForce GT710 2Gb RAM.
Cooling, a very large chip cooling set up with 12cm fan integral.
A Crucial BX500 SSHD.
Windows 10 Professional build 1909.
HDMI cable for graphics PC to monitor.
Primary Display 1440/800 32 Bpp
Windows 10 needed a lot of tweaking to bring the system requirements within range of the capabilities of the hardware. I will not go into the tweaks here as there are many good advices published and software available for smoothing out Windows 10's requirements.
Results;
FS 2004 with every slider and tweak at maximum and Terrain Mesh packs installed covering all of Australia (Terrain Mesh) and individual town/city scenery packs. About 14Gb.
FSX Steam Edition with almost every slider and tweak set at maximum with Terrain Mesh and round we go again. The only adjustments downward I have made are filtering set at bi-linear.
And, amazing, FSX SE running smooth as a politician's promises and in fact smoother than FS 2004.
A good SSHD here in Adelaide costs around $50.00 AUD. I do not know the cost converted into English Pounds, probably about 25 to 30 Pounds.
So, in all this, buy and install a Solid State Hard Drive and read up on how to strip Windows 10 of it's not needed "features".
Cheers,
Mark H.