No idea if you signed already or not, but here's my perspective.
You have to weigh your opportunity really.
you mention:
Would you miss out on the better opportunities you might get until September?
But the question is, would you have got better opportunities in these few months?
For example, someone with 10 years of experience and expert skill set will have a job search vastly different to a fresh graduate. You also have to factor in location and such.
No one can predict the future, but that's an important metric worth considering in my opinion.
Or would you sign the contract and then change your mind if you find a better opportunity (and deal with the legal consequences - if any)
Most contracts have that mentioned. Either a minimum duration of employment, or "you can quite with X months notice". Check your contract.
Would you tolerate your current employer for that long?
Depending on the red flags, you know your company best.
how can I do what is best for me?
Read the contract a MILLION times. Make sure it clearly specifies IN WRITING (none of that "boss said XYZ" stuff).
Make sure it clearly specifies your average work week, over time compensation, days off, sick days, PTO, salary, etc etc.
Also, check it for any non compete clauses, stuff like "If you quit, you aren't allowed to work for a competitor for X years" or "you aren't allowed to have a side job/free lance/etc at all" (yes some reputable companies do that).
There is no right and wrong, just make sure you read it well so you know what you're getting into.
