What Spoiler? None of the "spoilers" are relevant, and you lose a lot of context that improves the quality of your reading.
I don't wanna get into the nitty gritty here due to spoilers, so I'll just say this much.
I genuinely think that knowing Sinbad's backstory makes him less enjoyable in the main story (tho that's very subjective). It also removes an insane amount of tension.
The last arc of SnB is a massive spoiler. No two ways about it. Shinobu even lied about the events in Magi as to not spoiler it for SnB, which in and of itself should tell you in what order it should be read. To be more specific, I'm talking about why Sinbad is
half fallen. You might get away
with reading SnB up to the last arc, but definitely not the last arc itself. Heck, the end of the last arc ends at the start of the Balbadd arc (which is also majorly spoiled by
revealing Alibaba's background and relationship with Kassim) and not the beginning of Magi, so it's not even properly chronological.
Edit: I think the only saving grace of reading SnB first is that it would make the pirate arc far more bearable, as Madaura would be a more interesting character. However, as much as I hate that arc, it's just not worth the tradeoffs. Also, you just straight up aren't supposed to know a lot of the context that SnB gives.
Telling someone to read it before Magi because of that is like telling people to read books backwards, since the conclusion will help them understand the setup.