gnome – Keyboard layout shortcut interferes with Firefox shortcut

I am multi-lingual, and I use several keyboard language layouts.
I am used to using Ctrl+Shift in Windows to changue input language, so I want the same for my Linux. Default Alt+Spacebar is very inconvenient for me. Ubuntu’s vanilla settings app doesn’t allow Ctrl+Shift combination, so I had to use the Gnome Tweaks application, which allows more flexible options–including Ctrl+Shift for shortcuts.

However, I noticed that now Firefox’s shortcut to undo closing a tab (Ctrl+Shift+T), which I use extremely frequently, stopped working. It seems that as soon as Gnome/Ubuntu registers a Ctrl+Shift key combo, it stops caring if any other keys are pressed as well. It just switches keyboard layout with Ctrl+Shift+T the same way it does with just Ctrl+Shift.

This is a major quality of life issue for me. How do I fix this?

Thank you.