Some answers to your first question you can find here: https://madaidans-insecurities.github.io/guides/linux-hardening.html
For the second question about in what ways Secureblue do mitigate that you can find more here: https://secureblue.dev/features
The last question about usability, is very usable. If you use Bazzite you may have a similar experience. It is not like QubesOS that isolate all processes making it even not able to use a GPU.