If this were wotlk I'd explain you in detail how to handle it, alas I am unfamiliar with how spell flags work on any expansion past cataclysm as I didn't work with it.
In wrath mounts were auras, which counted as spell buffs, setting them to "usable in combat" and "usable while mounting" produces desirable results. Then you need to modify every spell you want to use whilst mounted to have the exact same spell flags. On certain cores its a favorable solution as it creates exactly what you want. How ever there is a catch. If the spell gets you into combat after casting, it becomes unusable till you leave combat. It just grays out and when you press it you get a warning "You cannot do that yet". So It has to be a spell visual at most that doesn't get you into combat. On arc emu tho, you can cast on a mount normally after the above mentioned tweak on spells.
An ideal solution how ever for you is to just use the .mount command and apply a creature to you, that way spells will work if flagged with the methods above.