Otherwise, you could edit screen game_menu directly, I guess. (Line 425-490 iirc)
I'm not sure how well that would play out, though, have not tested.
The style of Return button says
Python:
style return_button is navigation_button
style return_button_text is navigation_button_text
style return_button:
xpos gui.navigation_xpos
yalign 1.0
yoffset -45
Which means that it'll be "gui.navigation_xpos" pixels to left, on the bottom of the screen (yalign 1.0), but 45 pixels above that (yoffset -45).
You could duplicate the style, but change the x alignment to be the opposite; And move the Main Menu button from Navigation to Game Menu. May break things (but should not), but would be something like this...
EDIT: Consider reducing the spacing in gui.rpy:261, where it define gui.navigation_spacing = 6. You have quite a few buttons.
You might also want to reduce the offset in gui.rpy:400 where it define gui.navigation_xpos = 400 to use better the screen size.
View attachment 1284926