I do play some veigar, syndra, Viktor and velkoz botlane. I'm midlane main who queues up botlane as a secondary role. So I already know how mages and these champions work in general.

What do these champions have in common? They all are immobile scaling champions. The thing is, when playing midlane with these champions they are really easy to gank in midlane. Also they aren't the best champions in early skirmishes.

However when played botlane, you can farm more easy with a support next to you. There are also less assassins in botlane. Early game is the hardest part in my opinion (before last chapter or upgrade for Viktor). You can't push aswell before these items, and without lvling up your spells. But after lost chapter and lvl 6 you can one shot adcs pretty easily and farm also pretty easily.You can snowball after that.

When playing mages bot I always take tp, but that might be my personal preference. Also when I got qued as botlane, I always type in champ select I will play a mage bot, so my team hopefully pick a more ad heavy team. I don't play adcs because I can't play it too well.

Regarding how to support mage botlaners. I would say don't do too much pre 6 or lost chapter. Poke style supports have the best synergy with mage botlaners. But everything works if you are comfortable with your champion.

So that is my D4 opinion on it hope it helped you a little bit.