There are two theories, off the shelf then customize. or re-invent from scratch. Both has delivery curves. From scratch requires tons of knowledge, time, and expense but the benefit is exactly what you imagined. Customizing someone else's software that is mainly built could get you up and running in little time at the expense of groking your customization by reading other people's undocumented code.
This site took 3.5 years and 50 devs contributing with over 7k commits and that is not all the code that is in this site. The security module doesn't seem to be part of the open code base and I am sure there are others. For example they use a 3rd party to create the posts into a table of 751 posts that they used instead of writing it by hand. So lot's to do and consider. File downloads are usually handled by 3rd parties for most sites too, this is why sometimes the image server breaks. Few understand the bytes and bits and low level plumbing to avoid such 3rd party libraries.
Then you get into the conundrum of no one wanting you to succeed, no one to support you, the deep state stifling you with censorship and blacklisting, and others that sabotage your work. Not pleasant. I created one app with about 300k which was a static application, meaning that whatever it does is all that it does, and 300k on a dynamic one, which means how it is used is never defined - which was completed on the same budget (security and dev ops were still undone).
You need deep pockets, a great understanding to come out with anything novel to compete against the DS free funded works, and more than anything else determination of a mad man.
Hope that helps.