How to get marketo activity via middleware?

That’s a very broad question and without specific and detailed business requirements, impossible to answer in full.

I can say for now that you cannot push lines from the Marketo Activity Log, with all their details, to an external service as soon as they are added to the Marketo database. Via a webhook, you can push some details for some activities for which there are {{trigger.tokens}}, and you’ll need a separate trigger for each type of activity. But this approach will not scale beyond, to be conservative, 50K activities per day. Of course the real limit depends on the daily volume, and distribution over the day, of the activities you select.

Far more promisingly, you can pull log lines, on a schedule, from the Activity Log using the Paginated Activities or Bulk Export Activities API endpoints. Usually (though not always) Bulk is a better choice than Paginated. Again it depends on your detailed business requirements + real-life use.