Hi @cadu_bressan, 200 is indeed the maximum number of DMs that can be retrieved from the API.
It is possible to retain the content of DMs as long as the user is aware as stated in our developer policy (3.a.iv), but I assume this would introduce data storage issues that you do not want to handle.
Alternatively, you could store just the IDs of past DMs and call the API to retrieve them when necessary. This may or may not work for your use case.
I don’t believe there would be a third party app that could provide this service as providing content without our consent is not permitted.