(I'm non-technical) What is the best back end framework to use for my web app?

node.js which uses javascript The developer can then use Javascript both on the backend and on the frontend.

I did not intend to say that but I would say that

...adding, more exactingly, that php/laravel is _so_ popular and they are each so accessible that they should really be your first choice -- unless you have some compelling reason to go in a different direction. If you get to the point where you have funding, then your hired techie can worry about how to grow the tech. Adding, starting with one stack does not confine you to that stack. For instance, your web stuff can be in php/laravel, and your back end processing jobs/crons/queues/scheduled tasks can be in python, or hosted as js/node lambdas at aws, etc. Would be common. God knows what tech you will use to build your mobile apps -- about the only thing anyone could reasonably promise you there is that it _will_ be a disaster and it _will_ be the wrong choice no matter what choice you make there. So just start talking to people, documenting, probing, dealmaking, marketing, distributing, and building. Imo.

I misspelled Balsamiq too

Laravel is not perfect

For instance, their new version 8 massively complicated the entire ecosystem in myriad ways

But its still probably the safest best option

A big part of my decision would be if there is an existing saas/boilerplate/plumbing 'framework' available for the chosen platform

I'm confident in two or more

A couple/few related to php/laravel, and saaspegasus for python/django

I think there are certain truisms about this part of your journey

(Admitting I have zero basis to be listened to other than years of learning how not to do things, years of reading and listening to startup articles/pods, etc.)

Make a decision and move

There is no right answer

There is hardly a wrong answer

Do all the things that startup people say they will but still do not -- like stick to the M in MVP and not chase features without talking to paying users, don't write the native apps unless/until you truly need them (they are so much gd work), etc.

Welcome. I've been involved in this position before (came in as a PM/Tech in a startup which outsourced the development). It went well, but would be easier if the vendor were to stick with those requirements to ease maintenance/feature development down the road.