The earlier the better. None of them have exclusivity but they do have requirements on funding. If you already raised too much it can be problematic, or if you're already a paying customer them switching to credits might require extra work.
Sometimes they require you to have a VC or accelerator intro for the higher tiers but you may be able to get around it if you know the right people.
GCP has a regular small startup program that doesn’t require that you have VC funding or specific accelerator backing. It isn’t nearly as much, but I got $1000 on top of the initial $300 for my bootstrap.
I cant reveal much but they did change the rules and added a new smaller package. All 3 levels are technically available directly without an intro but again may take work to navigate the right relationships.
We document the levels directly on the site [1], though I think there’s at least one bug in the FAQ :). We try to make it clear that you can also apply directly.
Sometimes they require you to have a VC or accelerator intro for the higher tiers but you may be able to get around it if you know the right people.