It sounds like you are looking to build some automation, but are concerned about hitting rate limits. My advice would be don’t let the rate limits stop you. It’s been my experience here at Twitter that hitting the rate limits is not very common, especially for the DM experience. How certain are you that you will send 1000 DMs per day?
It should be an acceptable experience to automate a reply in DMs so long as that reply is valuable and expressly consented to by the user, and not just a standard message.