Will my visa application be rejected if it takes longer than expected to process it before I arrive in Thailand?

If you can’t enter Thailand without a visa (as many nationalities can’t) you’ll just have to wait until it’s approved.

Many people who apply for a visa can enter without one but they don’t get it in time so enter visa exempt, only to have to exit and re enter to activate the visa.

Many embassies will work on a “need to be issued” by date according to when you apply but that won’t guarantee you’ll get the visa in time. Sadly it’s just a wait and see process. You may get it in time but you may not.

They will give start date from the day of approval, don’t have to worry what you entered as that is your possible date of travel based on visa approval, wait for visa

Many people receive their evisa after they already arrived in Thailand because it took longer to process that expected.