Ideally it'd be cool if there were a default feature to set mount appearance upgrades per mount:
https://forums.elderscrollsonline.com/en/discussion/680430/hide-appearance-upgrades-per-mount-favorite-mounts-per-character/
But an OK solution in the meantime,
a keybind toggle to show/hide all three upgrades at once. Manual, but faster than menu-surfing.
Disclaimer: I know absolutely nothing about making addons, just guessing based on basic code logic

I thought it could be something short like this:
https://www.esoui.com/downloads/info455-AutolootToggle.html
But I don't know what the correct inputs are, or if this is even possible...
I found these refs here:
https://esoapi.uesp.net/100014/globals.txt
SI_INTERFACE_OPTIONS_HIDE_MOUNT_STAMINA_UPGRADE
SI_INTERFACE_OPTIONS_HIDE_MOUNT_INVENTORY_UPGRADE
SI_INTERFACE_OPTIONS_HIDE_MOUNT_SPEED_UPGRADE
Thanks to Feroc's Autoloot Toggle I got as far as this below, but obviously "autolootSetting" and "SETTING_TYPE_LOOT, LOOT_SETTING_AUTO_LOOT" needs to be replaced – Idk with what?
function MountUpgradeToggle()
local autolootSetting = GetSetting(SI_INTERFACE_OPTIONS_HIDE_MOUNT_STAMINA_UPGRADE);
if (autolootSetting == "1") then
autolootSetting = "0";
d("Autoloot is now off");
else
autolootSetting = "1";
d("Autoloot is now on");
end
SetSetting(SETTING_TYPE_LOOT, LOOT_SETTING_AUTO_LOOT, autolootSetting, 1);
end
SLASH_COMMANDS["/MountUpgradeToggle"]= MountUpgradeToggle
SLASH_COMMANDS["/mut"]= MountUpgradeToggle
If it's even possible to do all three in one keystroke, can that just look like:
GetSetting(SI_INTERFACE_OPTIONS_HIDE_MOUNT_STAMINA_UPGRADE, SI_INTERFACE_OPTIONS_HIDE_MOUNT_INVENTORY_UPGRADE, SI_INTERFACE_OPTIONS_HIDE_MOUNT_SPEED_UPGRADE);
?
Maybe this one's a totally different beast from autoloot and not that simple! Figured I'd ask anyway xD
.