Here’s what I came up with that is working for me. Thank you for the /run command.
#showtooltip
/run if IsUsableSpell(368896)then C_MountJournal.SummonByID(1563)end
/use [swimming]Subdued Seahorse
/use [flyable,nomod][flyable,mod:shift]Tangled Dreamweaver;Smoky Charger
/dismount [mounted]