!rant

Got a question since I've been working with ancient web technologies for the most part.
How should you handle web request authorization in a React app + Rest API?
Should you create a custom service returning to react app what the user authenticated with a token has access to and create GUI based on that kind of single pre other components response?
Should you just create the react app with components handling the requests and render based on access granted/denied from specific requests?
Or something else altogether? The app will be huge since It's a rewrite off already existing service with 2500 entities and a lot of different access levels and object ownerships. Some pages could easily reach double digits requests if done with per object authorization so I'm not quite sure how to proceed and would prefer not to fuck it up from the get go and everyone on the team has little to no experience with seperated frontend/backend logic.

Hi,
I'm not a ranty person so I never actually thought I'd post anything here but here it goes.

From the beginning.
We use ancient technologies. PHP 5.2, Symfony 1.2 and a non RFC complient SOAP with NO documentation.
A year ago We've been thrown a new temporary project. An VOIP app for every OS.
That being iOS, Android, MAC, PC, Linux, Windows mobile. With a 3 month deadline. All that thrown at 4 PHP developers. The idea being that They'll take it, sign the delivery protocol, everyone happy. No more updates for the app needed. They get their funds they needed the app for and we get paid.

Fast forward to today...

Our dev team started the year with great news that We'll most likely have to create a new project. Since the amount of new features would be far greater than current feature set, we managed to finally force our boss to use newer technologies (ie. seperate backend symfony4 PHP7+/frontend react, rest api and so on). So we were ecstatic to say the least. With preestimates aimed at a minimum 3 month development period. Since we're comfortable with everything that needs to be done.
Two days later our boss came to me that one of our most annoying clients needs a new feature. Said client uses ancient version written on a napkin because They changed half of the specification 2 weaks before deadline in a software made not by a developer but some sysadmin who didn't know anything. His MVC model was practically VVV model since he even had sql queries in some views. Feature will take 3 days - fixing everything that will break in the meantime - 1-2 months.
F*** it, fine. A little overtime won't kill me.
Yesterday boss comes again... Apparently someone lost a delivery protocol for a project we ended that half a year ago. Whats even better at the time when we asked for hardware to test we never got any. When we asked about any testing enviornment - nothing. The app being SEMI-stable on everything is an overstatement but it was working on the os'es available at the time. Since the client started testing now again, it turns out that both Android app does not work on 8.1/9 and the iOS app does not work on ios12. The client obviously does not want to pay and we can do little with it without the protocol, other than rewriting the apps.
It will take months at least since all of those apps were written by people that didn't know neither the OS'es nor the languages. For example I started writing the iOS one in swift. Only to learn after half of the development time, that swift doesn't like working by C Library rules and I had to use ObjC also. With some C thrown in due to the library. 3 unknown languages, on an unknown platform in 3 months. I never had any apple device in my hand at that time nor do I intend to now. I'm astonished it worked out then. It was a clusterf**k of bad design and sticking everything together with deprecated apis and a gum. So I'll have to basically fully rewrite it.

If boss decides we'll take all those at the same time I'll f***ing jump of a bridge.