[Xiaomi Mi 8 Questions & Answers] Crossvxm:
Welcome to the world of Xiaomi. Dated MIUI 9? HOW DARE YOU :p We have a wonderful theme store with tons of themes, something that cannot be said for other manufacturers that have added theme stores or theme apps (cough cough Asus). I'll give you a detailed summary of how things work.

UPDATES, MIUI, ANDROID:

A LARGE majority of Xiaomi smartphone users buy it for the chap price and expect it to work like every other Android smartphone; Android updates bring you features. That is totally fine, but many seem upset when their phone is still rocking an ancient version of Android as compared to other phones (both other brands and Xiaomi phones as well). For MIUI, this is not the case, and again, people still haven't realized this sadly. Xiaomi pushes features and improvements through MIUI versions, so for the most part, Android versions are irrelevant to us. Obviously some features from the Android version are important such as the upcoming support for notches and multiple cameras, but for the MIUI ecosystem, everything you need to for your experience comes in the form of MIUI updates. Focus on MIUI version updates first, unless there is something very important in an upcoming Android version.

FEATURES, IMPROVEMENTS, BUGS and YOU:

Your opinions DO matter, many of the features present on MIUI came from the voiced opinions of users like you and I. Second space, dual apps, themes, etc. Many of the are thanks to what we have made of it. This is the same for bugs. There is a feedback app within the phone which is in the Tools folder of most if not all official MIUI versions. Do provide screenshots and suggestions, they are listening, and I myself have seen bugs I have mentioned be fixed in later versions. Could have been me, could have been millions of people who noticed it as well. I think MIUI 7 was the most successful MIUI version out there in terms of overall stability, but I could be wrong

DEVICE QUALITY:

Obviously cheaper devices will demonstrate that they are cheap at some point. Most Xiaomi phones are solid companions, but the lower priced devices tend to suck at times. I owned the Redmi Note 2, and boy the battery life was dreadfull. Some other mid range devices of theirs have physical issues. I owned a Mi Max, it began to slightly bend like the iPhone 6 Plus. The Redmi Note 3 Pro had what seemed like paint on the antenna areas that was chipping off from just putting it in my pocket. The Mi A1 unfortunately had a major issue with the power button that many experienced, and after a drop in it's case, the screen began to lift slightly. This is not at all to scare you as things happen, and all of those phones were outstanding in terms of performance and battery life. Not to mention that all of these phones have been top notch perfomers (no, thats not a pun). Their flagships are pretty darn good for the price, same goes for quality in these. Another thing to note, flagships have better band support than the mid range or lower end (even if they have a Snapdragon processor). This is why I gave the Mi A1 up. I had LTE, but no data during calls, calls would not come though because of the lack of 3G or even 2G support for my carrier, no VoLTE and so on.

CUSTOM ROMS AND XIAOMI PHONES:

Not a good mix in my own experience, but there are some great roms. It's gotten better now that Xiaomi releases source codes even though it takes them a while, but usually the common issue is a lack of a variety of official roms, with many being buggy ports or so on. I hate LineageOS, but typically other roms use this as a base instead of being built from scratch. This is usually what I have come across Xiaomi phones, but who am I to talk, I love MIUI and have not touched a custom rom on Xiaomi phones since the...well A1 actually. MIUI is not for everyone however. There are some setbacks to some of their featues. The browser app for example is great, but auto playing videos gets very annoying, and it falls behind many other browsers out there (I use Chrome). Heavily modified manufacturer builds of Android such as MIUI usually runs better than custom roms though, so keep in mind that you may not get the 6 hour average of screen on time with heavy usage, or may experience more heat, etc.

THINGS YOU MENTIONED:

Now for some of the specific things you mentioned, MIUI always seemed to want to distance itself from Google, but it has gotten better over time. Look at the Mi A1 and A2 for example. I believe that these features just need better implementing. One thing I miss about stock Android is the newsfeed on the left side of home, like the iPhone. I don't understand why the MIUI launcher even had this section, it is useless as all it does is display things I could care less about, and I have to go to the Google app to access this newsfeed. Text to speech I believe now resides within the Google keyboard? Go to Settings > Additional Settings > Languages and Input > under Input Methods > Gboard > Preferences and enable Voice input key. Now you can tap the mic on the top right of the keyboard and talk to type. The text editing MIUI vs Google part has me confused. I believe it might depend on which app you use? If it's a Google app, it's a Google thing, if it's a MIUI app it's a MIUI thing maybe? If you could give some examples, I could investigate and see if there is a proper workaround.

BONUS:

A few secrets! VoLTE not working for you? Try changing your region to India or Hong Kong. Notifications not arriving on time or at all? This is a default battery saving featue, go to Settings > Battery & performance > under Battery Saver tap Choose apps and look for the app that's not receiving notifications (example Kik) and set to No restrictions. Missing your Facebook (or other app) chat heads? Go to Settings > Permissions > Other permissions and select the app in question. You will see Display pop-up window is disabled (red circle with an "X"), tap it and tap Accept. This is commonly referred to as drawing over other apps. I keep it off since it sometimes becomes a headache for changing other app's requested permissions on the fly. This is something that was added to Android after I believe Android 5.0 or so. It's a "security" feature, but if you just installed a new app lets say, have a Facebook chat head open, and that new app requests permission for contacts or so on, it may tell you to turn off the drawing app to change the permission, so I guess this explains why Xiaomi decided to keep it off by default.