I haven’t read all replies, but I assume people will give specific champions for you to try out, so I want to try and give you a process to follow, so you can find a champ you like.
First, sort by “this champion is being played as a support”. There may come a time to try off-meta stuff, but starting out is not the time. Since you mentioned Yuumi, I would advise to sort by “enchanter”.
Second, you should try to pick something with simple abilities, not someone with 100 words document for a passive. When reading abilities, if at any point you have to re-read it to know what it does, just move on.
Third, pick what grabs your interest. It is a game where we try to have fun, and playing something you like makes having fun easier.
Things I would suggest - Sonna, Soraka and Yuumi. A bit more complex are Milio, Nami and Janna.
I’m sorry, but can you share your math?
I will start with, the only good comparison is Shadowflame. Voidstaff and Rabadon have a different purpose, stats and cost.
That being said, HF deals more damage than SF only in 4 cases - against high HP and low MR targets, low HP and MR targets, and very high HP and very high MR targets, or if you have enough Penetration to deal true damage without the HF passive.
Since you are building Flat Penetration to burst squishies, you want the maximum damage on low to mid HP and low to mid MR targets, and while the margin is… kinda small, it is 100% in favour of SF. Furthermore, Flat pen works exponentially well the more Flat Pen you have. so having just more flat pen from SF is just better for the playstyle.
If you are against tanks, you may prefer HF, but tank-busting is not the job of a mage.