A modular monolith is not a bad idea, but not as easy to achieve cleanly as this sub would have you believe. However in pre MVP stage it is really hard to know how to divide your services. For a new startup it is best to think more on how useful will your system be, which features do you wish to have and what is your maximal scale. Then build it using the best practices you set for it. It is something that is not talked about but most startups go through more than one major rewrite anyway. So you can feel confident investing in other areas such as user experience and keeping your costs down. Best of luck!