So I guess this is a classic case if nit having the right keywords or knowledge to find what you're looking for. So thanks to u/enricodortenzio for kinda helping in this process ¯_(ツ)_/¯. Just if anyone wants to know a someting called hwui rounded corners shader, but corrext me if I'm wrong since I don't really understand much of what you can find under that keyword. The solution that works for me right now is substratum with curve https://play.google.com/store/apps/details?id=com.gaurav.curve imstalled and some people might say thanks captain obvious but you dont really find it anywhere when searching for round corners android pie etc and also I've never really used substratum and the only thing that I ever heard about it was back some time and it was about android oreos security policies basically breaking it. I just hope this post helps someone else find what they're looking for faster (unlike me since I've been running Oreo and up). Right now this is working flawlessly on my sm-a320fn running a recent build of LineageOS 16 rooted with Magisk. It's right there directly after startup without me having to do anything. Just like it worked back in the Nougat days ����.
Don't know the reason they moved that option. But you still can use the floating window adding the shortcut to the status bar. You can use the split screen by holding the recents button (if you use nav bar) or you can download this app to add the shortcut to the status bar too.