I added: GET /api/v1/athlete/{athleteID}/activities/{id1},{id2},.. which returns a JSON array of activities.
Add ?intervals=true to include the interval data.
The activities are streamed back one per line so you can parse them one at a time if you like:
[
{"id": "i2132160", "start_date_local": "2021-10-02T08:07:53",..},
{"id": "i2155441", "start_date_local": "2021-10-05T06:53:52",...}]