As stated you want to use any mesh system as your last choice not your first. If you are really insistent on mesh just go buy orbi boxes for every room and admire how pretty they are and ignore any performance issues.
If the walls are actually thin you might get by with a single centrally located router. Your internet speeds are pretty slow so you might get good enough signal strength to get your full capacity with one router.
Something with a number in the 1750-1900 range is the best performance for the cost right now.
If this is not good enough you then need to add more radio sources. The next step would be to place a AP on the second floor. A AP is best connected to the main router via a ethernet cable. If that is not...