anas mattar

I understood from your question your website doesn't has alot of traffic, so you can start building it as monoliths architecture and then if your website has more visitors and alot of traffice in this time you can start thinking for spliting the website to multiple services.

It's better to use Laravel with Vue.js and also laravel is very lightweight and speed performance.

I don't recommended to use WebRTC for group meeting because when you have a lot of participants of a meeting so in this case, it will happen hung in the connection because the stream pass over http, also webRTC support maximum 256 participants of a group meeting. webRTC is good for calling peer to peer. you can use zoom program and then integrate it with your project via api or embedded system

my advice , you should answer me for this question, what do you like to work: web base or mobile native or cross platform. if you like web base you should choose PHP or ASP.net or Node.js or if you like mobile native you should decide Android or IOS platform and else if you like cross platfrom you should learn Flutter with dart language. thanks

Since you're team is already using JavaScript, there are alot of examples and open source projects written with NodeJs, so I preffer this language in your backend application and also I am recommended using Mongo DB with It for saving data in it, and also for your frontend application I am recommanded using VueJs.

That's depend on your experience if you are very well in C# you should start using the Technology that's you know and like it.

start with a small project and only one language and then try to add new features to it.

I prefer to use Magneto because it open source and has a lot of extensions in it so it's so faster for building a website

I prefer to use Node.js because you have experainse in it and also you can do anything for this language.

for what technologies you should use, this is depend on what technology do you prefer? your should think best structuing for your code because each API vendor has different to a nother one so it's better no merege code vendores together. your code must be using SOLID principle pattern and some design pattern such as Factory Pattern