About rom, I recommend this:
https://xdaforums.com/grand-prime/development/official-xenonhd-7-1-2-galaxy-grand-t3614600
Custom roms mean roms made by users, not by the
manufacturer, and stock roms is roms made by manufacturer (the rom you're using is a stock rom).
The link I provided above is a custom rom. Its Android version is 7.1.2 (too much for you, eh?) and it has better performance than the stock rom that helps you play PUBG smoother (at least comparing with stock rom).
But, you know... although it's an "official" rom, it still contains sone risks. If you don't want to risk, then choose your country and varient to download stock rom from this site:
http://samsung-firmware.org/downloa...l Sim)/0m03/SER/G530HXCU1BOI1/G530HSER1BOI1/.
But I think the first link I provided you is worth trying. It has been tested and has been marked as "official", so I don't think it will cause any problems.
If you want help, just pm me