surely no one will name all the parameters, without the preliminary load testing of the application, otherwise it is, to put it mildly, a delusion.
I think we need a cluster with simultaneous calculation of the application architecture, which will allow us to scale as uselessly as possible in the future, and start with minimal resources.
summarizing all of the above, you need: front-ends for balancing -> backend for processing <- db and caching servers - a typical cluster.
all this wealth will cost about 4-5 thousand dollars a year. it's just a lease, with your personal full customization of it all.
Well, in the end, it's hard to call hosting, in the true sense of the word.