The Post-Money SAFEs, as they currently are worded, do not have your prior SAFEs dilute with raises from subsequent SAFEs.
The Pre-Money SAFEs are more founder friendly. But investors will likely push back on this and not accept them. If you have control of the round, you can issue a Pre-Money SAFE which is more founder friendly.
With the Post Money SAFEs there are some things to address this, but if you can do one Post Money SAFE it's preferred or raise the valuation of the Post-Money SAFE when you issue it.
Let us explain with a few examples.
Let's say you raise $500k on a 5m Post-Money SAFE. That $1m represents 10% of equity. And 10% of dilution.
And then let's say you do an equity priced round at a $10m Post-Money valuation, and raise $2m of fresh cash. This $2m represents 20% of dilution. And this will also dilute SAFE equity by 20% -- so the 10% from the SAFE amounts 8% of dilution after the priced round because the SAFE is diluted by the equity round.
Now if you structured that $2m equity round as a 2nd Post-Money SAFE instead of an equity round, the 2nd Post-Money SAFE will NOT dilute the first SAFE. If you did your priced equity raise after this -- say you raised $5m of fresh cash at a $25m Post-Money -- the first Post-Money dilution of 10% would only be diluted by the 25% equity raise, but not by the 20% from the 2nd Post-Money SAFE.
This is why some advisors will tell you to only do one Post-Money SAFE, or to structure the 2nd Post-Money SAFE as a priced round.