From my experience, stock options are priced based on start date(correct me if this is not standard). While the price changes less frequently for private stocks compared to public stocks, they still change (Typically through investment events as you mentioned). Thus, if this particular startup raised a seed round in the coming months, then the value of the stock options would not rise as op mentioned - "The option % they've offered me would only rise in value" - since the pricing of the stock options would be different.