There are a lot of information gaps in what you described, and some of the app upload description seems a little unusual in the approach, but maybe the following will help?
CSV is a plain text format, it likely has a lot of repetition within certain elements. That means that they are probably easily compressed to a much smaller size through, for example, ZIP compression.
Shortcuts is one option on an iPhone that can ZIP and upload files to a shared Dropbox folder. This could also be used to enforce naming standards, stop duplicate uploads, etc.
In addition, anyone uploading data should attempt to do so over a suitably fast Internet connection.
If the data is on Apple Watch for the numerous apps you are apparently using, I would hope that there is something within the apps to allow for export/download to the associated iPhone. If not, you are going to be entirely reliant on any upload functionality within the watch app and you will need to discuss transfer issue with the developers of those apps.