This actually did the trick. It is not sufficient to remove the font definitions from theme.txt and to add GRUB_FONT to /detc/default/grub.
I had to add the font to theme.txt
/boot/grub/themes/EndeavourOS/theme.txt
# cat theme.txt
# GRUB2 gfxmenu Linux EndeavourOS theme
# Designed for any resolution
# Global Property
title-text: ""
desktop-image: "background.png"
desktop-color: "#000000"
terminal-font: "Hack Regular 24"
terminal-box: "terminal_box_*.png"
terminal-left: "0"
terminal-top: "0"
terminal-width: "100%"
terminal-height: "100%"
terminal-border: "0"
# Show the boot menu
+ boot_menu {
left = 10%
top = 10%
width = 80%
height = 80%
item_font = "Hack Regular 24"
item_color = "#cccccc"
selected_item_color = "#ffffff"
item_height = 24
item_spacing = 16
selected_item_pixmap_style = "select_*.png"
}
# Show a countdown message using the label component
+ label {
top = 82%
left = 35%
width = 30%
align = "center"
id = "__timeout__"
text = "Booting in %d seconds"
color = "#cccccc"
font = "Hack Regular 24"
}
Thanks for helping!