Exchanging my iPad Pro 2021 for Mac mini

When purchasing a new device directly from Apple, such as your iPad, you have an 14-day “unconditional returns” period in which to return an unwanted device for exchange or refund. Upon expiry of the 14-day window of opportunity, other than for warranty purposes, the opportunity to return at no cost is lost.

Apple do offer a trade-in, where qualifying devices can be returned for credit against an immediate purchase - or for an Apple Gift Voucher of the trade-in value.

Apple Trade In - Apple (UK)

You should note that achievable trade-in values are always substantially lower than the original purchase price - but offers an opportunity to recoup some value; as you have discovered, trade-in of a nearly new device is unlikely to economically attractive.

Your remaining option is to sell your device privately - where you may be able to achieve a better value than trade-in - but to be an attractive option to a private buyer will obviously be less than the full retail price that you paid only a few weeks ago.