You will want to avoid hitting the rate-limit as that acts as a temporary ban, so will get your maximum throughput over time if you can stay under that limit.
To do this - there are a few inbuilt parameters you can use. You can find them here:
I am spesifically talking about the bwlimit and max-transfer parameters.
bwlimit will cap rclones bandwidth, so you can easily math out what is a safe 24/7 speed (I think around 8.5Mbit?). This might be an simple and easy solution if if it fits your needs.
max-transfer will actually error-out rclone when it hits a spesific size limit and stop transfers. Keep in mind this is limited to a single rclone instance - so it is not so useful if you do many separate sync operations during a day because rclone won't remember from last time. If all your quota is swallowed on a single big daily sync though then it will be perfect.
Hope that works for you. Hopefully we can get more elegant solutions in the future. If google would just let us poll how much quota we have left this would be simple - but of course they want to be vague about it...