Couldn't you just leave a standing limit buy order? (assuming you have fiat balance).

There are quite a few exchanges now that offer DCA, but as far as if any do this type of a limit order, I wouldn't know without researching each. The following blog post has a list of exchanges with DCA:

Dollar Cost Averaging. The answer to the question “Is now a good time to buy Bitcoin?” https://cointastical.medium.com/dollar-cost-averaging-the-answer-to-the-question-is-now-a-good-time-to-buy-bitcoin-a84e518f50f0