I have a similar model i.e. Asus ROG GL503VD FY126-T
FYI, I've replaced the original 128Gb Samsung PCle3x4/NVMe, M.2 SSD with a 500Gb Samsung SSD, and added an extra 8GB of RAM to the available slot. No problems at this time.
1 .. SSD upgrade with 500Gb Samsung 970 EVO Plus NVMe M.2. This SSD was placed in a 'ORICO NVME M.2 SSD ENCLOSURE (10GBPS)' for cloning of the original SSD to the new SSD (cable supplied with the enclosure). I did the cloning using Tools/ Clone Disk in Acronis True Image 2018, and just used the default settings. From memory it took about 10 minutes or so to clone. I then just removed the back plate on the laptop and replaced the old SSD with the new one. The laptop booted up fine, and has worked well since.
2 .. I added the following RAM card to the extra internal memory slot.. 'CRUCIAL CT8G4SFS824A 8G DDR4 2400 SODIMM CL17 SINGLE RANKED, APPLE AASP'. Again, no problems after boot-up. Note that the original Hynix (Hyundai) 8Gb SO-DIMM DDR4 1Rx8 PC4-192005O DDR4-2400 RAM card is still installed. It appears to have the same specs as the Crucial card (or at least close enough).
3 .. The items were purchased from Centrecom (Vic), and the 'capital letters' are to show the part descriptions on the invoice.
Hope this helps.