Upcoming vs forthcoming

Hello,

I've done some research on what the difference in meaning between these two words is, and have mostly found that they are interchangeable. However, I have come across a few comments on different forums indicating that 'forthcoming' is more representative of an ongoing process that will culminate to some endpoint in the future, and that 'upcoming' is more specifically referring to that point in time. I didn't find anything official indicating those assertions, though. Does anyone here have an opinion on the matter?

Also, I've discovered that 'upcoming' is not informal, and is certainly more commonly used today. However, some comments I've come across have stated that 'forthcoming' sounds more formal and academic, and should thus be used in formal writing instead of 'upcoming'. Does anyone have any opinions on this matter as well? And, overall, which word is best to use for formal writing, all things considered?

Thank you!