Thinking of [name_u]Toby[/name_u] [name_m]Sinbad[/name_m] [name_u]Walker[/name_u] - my partner thinks the gentle [name_u]Toby[/name_u] offsets the more aggressive sounding [name_m]Sinbad[/name_m] to sound more punk.
I’m half [name_f]Indian[/name_f] with Persian roots, so [name_m]Sinbad[/name_m] is, to me, an adventurous sailor who always crashed his boat but made friends, a fortune and somehow always got home. That said, I understand [name_m]Sinbad[/name_m] is literally two terrible English words stuck together.
Oh and it’s worth noting that this name as for me, not a poor child who has to go through school introducing myself.
If the name is for you, you get to decide on your own if it’s too much middle name! With a baby, yeah, I think you have to try to consider what it would feel like to go through life (and school!) with a certain name, and be open to feedback on whether that name would be difficult to live with (at least that’s my naming philosophy), but if if it’s for you, I think it’s probably best not to worry about external views, and pick what feels right to you. What can you imagine yourself putting down on your driver’s license, on job applications, in ten years from now, twenty, etc.?
So, it’s interesting that you break down [name_m]Sinbad[/name_m] as Sin and Bad–I’m of [name_m]French[/name_m]/American background and never thought of it like that. Only as the adventurer/sailor/story character! And also this comedian that was famous when I was a kid in the 90s (Sinbad (comedian) - Wikipedia)
Also, I live in [name_f]France[/name_f] and I recently met a man (late 20s? early 30s?) named [name_m]Sinbad[/name_m], so you wouldn’t be the only [name_m]Sinbad[/name_m] out there!
That said, if you’re looking for other people’s opinions, to me, it’s a bit much, precisely because I would expect most people to make the connection to the character from the stories, and I tend to shy from names that are so closely linked to a single personality/story/etc, but if that’s the story you want for yourself, and you won’t be annoyed at people noticing the connection, then that may not be objectionable at all for you. It could even be a cool conversation-starter. Also, you never have to tell people your middle name if you think they’d be annoying about it (except maybe places where you have to show ID?). [name_u]Toby[/name_u] is a great name, and a very familiar one, and you could always leave it at that, with a secret, punk-rock nod to a wild adventurer in the middle.
Good luck with your choice 
[name_m]Sinbad[/name_m] makes me think of [name_m]Sinbad[/name_m] the sailor. I think pretty much everything works in the middle name slot, so it’s not a bad choice if you like it.