This is a useful reference about SAFE Notes.
In essence and in the ordinary sense, as SAFE notes aren't a loan, and investors don't receive any sort of interest or payments nor do they provide dividends, there is no guarantee that it will ever convert into equity which in effect, for revenue purposes could mean that that there is, in fact, no consideration at the time of issue.
So, in answer top the specific question:
when do you pay tax?
I would agree with the view expressed:
During the financing round when SAFE convert to equity of the company
I would suggest that to be the correct position and the only mechanism for determining the value of stock on which payment for services has been made which can generate a tax liability, albeit delayed. The OP may never be paid for the service provided.