I'm sorry, but I think you are mistaken about tlp not being installed with Gnome on Manjaro.
Generally TLP is installed by default on any laptop with Gnome on Manjaro. On new installs on desktop computers TLP may not be installed, but with a laptop it likely is.
You need to double check if tlp is installed (as it most probably is). Then you can mask tlp, or simply uninstall tlp to see if the battery issue goes away. That is not a proper fix though, as a power manager (tlp or other) is required on a laptop.
The best solution is to go through tlp's battery power settings and systematically change any setting regarding battery power and bluetooth/wifi. You can install tlpui if you do not want to edit tlp's root config file.
Be sure to reboot after changing only one setting at a time. Then check for results of the change after rebooting.
Many of the Ralink/Mediatek adapters currently seem to be having issues with their driver. The best chance for a fix is to test different kernel models. It might also be a good idea to try a real time kernel in this situation.
Testing older versions of the linux-firmware package may also be a possible fix for wonky wifi. You can search "downgrade linux-firmware" if you wish to test this method.
You can also try this fix:
Disable Network Manager's Power Saving
Disable Network Manager's WiFi power saving features with the following command:
echo -e "[connection]\nwifi.powersave = 0" | sudo tee /etc/NetworkManager/conf.d/default-wifi-powersave-on.conf
A setting of "0" will totally disable power saving features in Network Manager for your WiFi adapter. A setting of "2" or "1" will be less aggressive, but still leave power saving enabled. A setting of 2 is sometimes enough to eliminate problems, while decreasing battery life very little. A setting of "0" does increase power consumption marginally, but not enough to be a big issue.
Reboot after creating the /etc/NetworkManager/conf.d/default-wifi-powersave-on.conf file.
This change will marginally increase your WiFi's power usage, but it may help with your issue. If you wish to revert the changes completely then delete the file: