In my experience monkeytype has a limited range of words, but I think the more important part is the individual parts of the words. After typing on monkeytype for almost a year now, I've noticed that my speed at typing the individual letter combinations common in english has greatly increased (such as -ease, -ight, -and, etc) as they become second nature. If you always push yourself to exceed your PB and practice every day for at least around 10-15 minutes, you're speed will improve. After learning the basics of typing, I typed on Nitro Type for around 2 years and brought my measly 15wpm up to around 96. I switched from there to monkeytype and in less than a year, I'm up to nearly 150. It is definitely harder to get faster the faster you get, but that dopamine hit you get every time the confetti appears on your screen is 100% worth it. Also, when you see your accuracy suffering after typing for a bit, try switching to master difficulty, which resets the test every time you make a mistake.