I also brought a ring with me to Mexico to propose to my Mexican girlfriend (we got engaged in SMA) - good luck and best wishes!

Over more than 16 years of travelling from the US to Mexico I have never declared jewelry. I have received the dreaded "red light" for additional inspection at Customs more times than I can remember, including (especially?) when arriving during the Christmas holidays. They have never asked about jewelry that was clearly being imported as gifts (e.g. in those iconic baby blue or or dark red/gold boxes) or even about wristwatches in boxes (although I do keep purchase receipts with me in case they decide to collect duty - you dont want them arbitrarily setting an inaccurate, highly-inflated value). The only time I had any issue was having a 3rd laptop (I was bringing in a new one a family member purchased remotely). In that situation I was able to use my phone to establish dutiable value - which was a little bit less than the receipt I had but didn't show them since they agreed to to the lower online price).

So, of course YMMV, but I think it highly unlikely you will have a problem.