Should I replace my mid-2015 27" iMac now or should I wait?

There is no clear answer to your question. It's up to you. Keep in mind that all computers eventually fail. Due to the age of yours, it may continue to work for years, or it could start filing tomorrow. So, the main thing is to have a good backup. That way, if it does fail before you get a new computer, you are covered and can then buy a new one.

do some research before you jump on it there is not much you can upgrade or even do in a apple silicon mac , they are stuck the way you config them so adding extra ram like the older ones is out, so make sure if you jump get what you need at order or you are SOL. i myself would go to a costco and buy the 27 in intel based one they still have and offer the apple care on if i was going to upgrade. but i myself see no need my late 2015 imac still does what i want it to do. and i bought it referb so i have a warranty from upsie on it.

If you have no pressing need, I would wait to see what the Apple silicon iMacs have to offer.