https://developers.tron.network/reference/rate-limits
check this doc on the rate limitation. besides this total request amount limitation, it also limited the request amount per second. which is 15 requests per second. make sure your request is less than 15 per second otherwise it will run into 403 or 503.
Build up your own fullnode is the best choice if your request amount is huge. you can find the fullnode set up guide at : Helpful Links for Developers
ANKR recently just supported TRON network, ANKR is also an option.