You could next season with the system riot plans on implementing.
But none of your mages will function as a sup.
Velkoz used to be able to. But since he relies so heavily on expensive AP builds to get shit done, he's far less effective.
Personally I'd go vel as others have said. But a simpler alternative to velkoz is ziggs, who has a very similar kit but without the true damage.
Velkoz doe simply far more reliable than xerath because 3 of his hit boxes are large and fast. Whereas xerath's w is highly telegraphed and his ults hitbox is tiny and difficult to land.
Based on your interest in vel, asol and ahri. Purely based on observation and knowledge of us low elo players, it appears you plan on maining high win rate champs, as all 3 of them are up there.
If you plan on mid, start small. It may seem cheap to win as annie, ziggs or malz. But you'll have a much easier time finding mistakes, maintaining a reliable performance and in turn improving.
Because you won't be able to view the only mistake as "missed skillshot".
Just mah 2 cents.
Glhf bae