They’ve recommended in the past to create a pool of machines, then just start/stop them as needed.The only cost for stopped machines is a minor rootfs storage cost.
In the first link they’ve also stated that there may be a time in the future where that may not be required (to manage your own pool) which seems to imply that rate limits might be eased.
In regards to rate limits, they have posted a few times that its a hard rate limit. I don’t remember all the threads where they talked about this but there are more if you’re able to do some searching.