I do think you're right about how strong Syndra is, but I don't think it's much of a big deal in the context of the greater plot. When it comes right down to it, on a meta-narrative level, Syndra (or any other character) is exactly as strong as the story needs them to be. She might be OP, yeah, but I don't think it'll ever really harm the narrative too much.
Trying to discern a champion's "power level" or something like that is something I've always found to be sort of a wild goose chase, because of a few reasons:
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League's story is not a free for all. The idea of "X champ can beat Y champ up ergo X champ is better" isn't applicable because not everything is a straight-up fight. Syndra's extreme magical power comes at a massive lack of any allies or diplomatic/personal skills to back her up. She doesn't show any particular ability to outthink or outplan people. For all her immense magical power, there's not a ton she could do if say, Jhin rigged her bedroom with traps she's not expecting, or she gets massively overwhelmed by the numbers of the Kinkou.
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Static power levels make for dull storytelling. If there is a conflict between, like, Gragas and Karthus, that occurs in the story (I'm spitballing, this particular case will...probably never happen.), the outcome of that conflict is up to the writer. If it's determined by going "Well, Gragas has 2231 Power Points and Karthus has 2100, so Gragas wins", that's boring and uninteresting. Analyzing the actual strengths and weaknesses of these characters, the setting, external circumstances, etc. and writing the outcome to mirror that is the job of a writer, and it's something we can see Riot doing in a few recent stories.
Hecarim might be a nigh-unstoppable undead hate-machine, but with a little backup and the use of a sacred relic, Illaoi was able to make him turn tail in Shadow And Fortune.
Ta'anari was an incredibly powerful Darkin warlord. His death came at the hands of a conniving traitor with a knife and good timing. She didn't outfight him, she manipulated and outplanned him, waiting for the moment he was at his weakest to end his life.
I just noticed Scathlocke joking down the page about the whole "my dad could beat up your dad" trope, which I think is a good way to view it. We can argue and debate over which champions are the most powerful (And there's nothing wrong with that.), but in a storytelling context, "power level" always plays second-fiddle to...well...story.