For a startup, boring tech stacks are absolutely the correct choice. Using an opinionated framework like Django can be a very good idea for the same reason - it provides a structure which you just have to follow without too many decisions to make, and any new hires with experience in the framework can hit the ground running to a larger extent.

The only exception is if you are doing something where your tech stack is integral to your product (e.g. you need crazily high performance or scale or something from the get go).

Split out the front ends into separate services but leave the back end as a monolith, just try not to get logically separate parts too entangled so you have a decent chance of separating them later if and when "later" arrives