Hello sorry for not answering sooner I managed to solve entirely my problem by installing mainline OpenWRT.
I know it won’t be an answer to everybody still having the issue as it’s quite difficult to install (you need to use u-boot to flash) and you lose the user-friendly interface of gl-inet.
So it’s definitely not a hardware issue but some kind of bug with the gl-inet modified openwrt. Also I noticed that my load average is now much lower than with the gl-inet firmware (it’s now below 0.10 most of the times when it could go up to 13 with the gl-inet firmware).
for installation instructions I used the following links (luci won’t be installed by default) :