This is surprisingly complicated. None of the following is advice, but rather pointers to discuss the surprisingly complicated issues with your professional advisors.

You may have nexuses ("sufficient connection to trigger sales tax obligation") in other US states even in absence of an office, for example if you have an employee working there remotely. Some states also consider actively selling within the state to establish a nexus, and consider a website accessible to be actively selling; this is a talk-to-an-accountant sort of question.

You unambiguously have a nexus in California and are subject to California's laws regarding sales tax.

You likely do not have to charge California sales tax on the downloadable software; c.f. https://www.boe.ca.gov/formspubs/pub109/ Your accountant will likely point you to Cal. Code Regs. tit. 18, §1502(c) with regards to the SaaS part, deeming it to also be untaxable.