The best way to avoid issues with volte, wifi calling, etc, is to buy a phone directly from the network. No excuses if it doesn't work.
About "Nothing", their focus seems to be more on the "experience" than on things like software updates. Not a problem for most, but we should keep in mind that they're not Samsung, Google, OnePlus, etc. For now it's a niche brand with a niche product.
If you're not going to do anything advanced with Android, maybe an iPhone could also be a good choice? It's not for me, but an iPhone 12 from 2 years ago gives you 5G, still another 3 years of support to go, and wifi calling should work fine.
Regarding Xiaomi, I don't know if it's because they're more mainstream now in the west now, but I feel that their cheaper phones are having more failures now? Some have widespread issues. See the Poco X3 Pro/NFC which had a PMIC/motherboard failure on many devices, for example. Mine was working fine for ~3 months, then I gave it to my mother (restored everything to stock MIUI, locked it, etc) and the sound died after less than a month.
Their flagships might be better, but the mid/low-end devices are like the fast food of mobile phones. I only buy them for the price/hardware/custom ROM support.