Follow-up:
OK, having done some digging, I can confirm that the rate limit you’re seeing is expected. The TON API is currently primarily provided for the use of e.g. Ads API partners needing to upload tailored audiences, rather than being intended for image access on DMs.
Unfortunately there’s currently no way to modify this - appreciate this isn’t the response you’re hoping for (and I noticed your #HelloWorld request for better docs on this too) - but that’s the current state.