You're highly unlikely to find a laptop that meets all this nowadays. In recent years, the trend has heavily shifted towards low-power and slim/light platforms.
Today, laptops with extra slot for a 2.5" hard drive/SSD usually are 15.6" or larger models. Those don't fall into the slim and light weight category, more into the bulky and heavy one...
Upgradable dGPUs, kind of deprecated these days; it was something usually restricted to mobile workstations in the form of MXM modules but I don't think this kind of technology remains much in use today. And even in the days where this was available in laptops, MXM modules were often limited to specific offerings by the manufacturer due to proprietary design. So upgradability was very limited and usually to 2 to 3 dGPUs, 4 if you were lucky..
Examples: Dell Precision M4800/M6800.
https://dl.dell.com/content/manual32906200-dell-precision-mobile-workstation-m4800-owner-s-manual.pdf?language=en-us&ps=true
The last 14.1" Dell mobile workstations were the old Merom/Penryn C2D-based Precision M2300 and M2400. Former was nothing but a Latitude D630 with a better nVidia dGPU (Quadro FX 360M) than the standard/regular one (Quadro NVS 135M). Latter was the same as the Latitude E6400, also with a better nVidia dGPU (Quadro FX 370M) rather than the standard/regular one (NVS 160M). All subsequent Dell mobile Precision workstations are 15.6" and 17".
https://en.wikipedia.org/wiki/Dell_Precision#Dell_Precision_Mobile_Workstations
Keep looking for the rare beast!