5540 SSD

One of the two slots will take a 2280 SSD.  The other takes only 2230.

There is a slot for a 5G card so not sure where to put the SSD.

You mentioned a Latitude 5540 -- documentation below.

Or is this the old Latitude E5540 from 10+ years ago?

https://www.dell.com/support/home/en-us/product-support/product/latitude-15-5540-laptop/docs

My machine is 13th gen Intel, its a 14 inch laptop

That would be the Latitude 5440 -- not the 5540, which means you are indeed limited to an M.2 2230 drive.  The system you original claimed to have is the 5540 -- which is 15" and does have space for two M.2 drives (one 2280 and one 2230).

This is my <Private data removed from public view. DELL-Admin>, warranty started Jan 2024

I notice when testing a 135W USB power brick that my machine performance improved dramatically. This shows the OEM 65W PSU is grossly inadequate,

If it's the Lenovo adapter you mention in the other post, it actually doesn't, since the system won't draw more than 90W from a non-Dell adapter.

There's no way that system will draw more than 65W running flat  out -- and it won't do that for very long before it throttles.  Yes, the CPU may draw 55W but that's only if it's mounted in a system that can sustain that power level, and only if all but one core is shut off.  In other words, it's a theoretical limit as it is anything resembling reality.

MY machine shows 90W suggesting the OEM 65W is underpowered

For example, 65W fuzzy donut is 12 fps, 90W is 18 fps which is very material