Good to hear you got the Play store working; was it the fix I mentioned above that worked?
Regarding changing the default SMS app, apparently others are strugging with that (some want to change to Google messages for group messaging), and they can set it as the default, but the phone reverts or overrides this back to the original factory installed one. I'm not sure if there is a solution yet, but check one of the other threads. I tend to try not to fight the phone, and anyhow don't use SMS much so not an issue for me; and I have Whatsapp/Wechat/Telegram, etc. just as separate apps I use.
Regarding the settings for full screen for some apps being rotated to vertical when you open them; you can install Rotation apps from Google Play that force rotation (see this thread -
https://forum.xda-developers.com/t/...ps-and-force-rotation-to-landscape.4386573/); but I find that it is best to just set those apps to 4:3 aspect ration rather than full screen, as they generally don't need or use the extra full screen width anyway; and then they just work correctly when the phone is either folded or unfolded without the Rotation apps.
Again, look at the other threads for Nova and adb, but so far those who have installed it using adb find it has quirks (like if you swipe up on the home screen it keeps flipping between Nova and the original launcher, and the dock is on the right edge instead of bottom); but this is another one where I think is it better not to fight the phone, and the default launcher is quite good if you give it a chance.